There are many things that can encourage hesitation. None are unrecoverable though, and some are avoidable.
Asking the whole group
Ironically, one common source of player hesitation is asking the group as a whole — when you do that, nobody in particular has to answer so everybody has permission to keep letting someone else answer. “What do you do?” is a fine question when it's obviously already addressing one character, but when it's the group, it works better to pick someone to answer (at least at first) based on whose next action is most interesting* in the situation:
Breaking that jewel out of the floor made a lot of noise. You hear footsteps coming toward you at a run from the far hallway. Fighter, what's your reaction?
As a bonus, if the answer is non-committal, you can then ask another likely PC what they do / what their reaction is, as if you meant to do that all along. “Okay. Bard, the footsteps sound like they're about to round the corner. What are you doing?” (In move terms, let's say the Fighter responded with “ready my sword” or something else OK but not moving the action along, which is doing something and then “looking to the GM”; so you got a GM move and used show signs of an approaching threat again.)
Who you ask also will tend to (but not always!) bias the answer in a particular direction. If you ask the Fighter, you'll probably get an answer that is fighty or preparing to be fighty; if you ask the Thief you'll get something more sneaky, perhaps; if you ask the Bard you might get something more social. It's not a guarantee, but you can nudge the action toward a certain directly this way. As a bonus, the hint that it's a Fighter-y (or Thief-y, or Cleric-y, or whatever) situation can signal the players a bit more about what they're facing, and may relieve the players' indecision enough for someone to leap forward with actions.
So: ask a specific character what they do instead of the whole group, since asking everybody can often be the same as asking nobody.
* “Interesting” can be “most obviously relevant” (i.e., the Fighter's response to a hostile threat), but it can also be very interesting to ask a character who is not obviously the lead for certain situations, like asking the Bard what they do in the face of armed hostiles. Mix it up, and the game will be more varied and interesting.
Alternatively, recovering is totally possible
Avoiding it is easy, but in the moment you might do it anyway and ask the whole group, or maybe that just seems like the most sensible thing to do at the time. When the whole group hesitates significantly to answer a question, give yourself permission to wait. The in-game action might be fast and furious, but the out-of-game action is a conversation, and sometimes some thoughtful silence improves conversations.
So although it's likely not your first reflex, relax and let the silence draw out. If you don't seem rushed, that can actually quickly wash away the group's collective hesitation and give people permission to stop thinking “ah, I don't know, I'll let someone else answer!” and sit back and actually create an answer they might offer to continue the conversation.
So letting them take their time to move the conversation forward is sometimes just fine. Give yourself permission to have quiet spots, even if the in-game situation is tense. (Believe me, Dungeon World sessions can be an exercise in sustained intensity, so a moment of quiet in the middle of a tense bit of action can actually be a welcome change!)
A player freezing when put on the spot
This one isn't so much something to avoid, as something to be confident the game can handle. The recovery can be pretty much the same: let the player collect their thoughts if they have to, by patiently giving them a moment of quiet to back away from the on-the-spot panic and thoughtfully engage the question.
Often (more often than you might think), that won't detract from the pacing of a situation at all, and will do the trick. Sometimes the player is really stuck though, or the pacing really does want an answer nownownow. In that case, ask for confirmation, in the form of tell them the requirements or consequences and ask:
Wizard, are you just hesitating? If so that's OK — we can see what someone else is doing in reaction to the sigil.
They get a chance to decide no, I am not just standing there! and say what they're doing instead. If so, cool! The game proceeds. They also get a chance to instead beg off the question, but that also results in an in-game action (hesitating, lost in thought or study of the phenomenon) and that's a tiny bit of forward action. (You also get the chance to practice a rare low-pressure consequence — not every GM move has to be world-shattering.) While they hesitate in-game, you ask someone else:
Ranger, this glowing rune on the wall is obviously not natural, but — worse — it's feeling more intensely unnatural by the second. What do you do?
Thief, you've heard of wizard sigils through professional contacts. What's the worst thing you've heard happen to a burglar who ran into one?
Giving another character a chance to react lets you paint what they're all seeing through a different character's perspective, giving them all a bit more context for their decision-making.
It also just gives the group another opportunity to do something interesting before “sudden DOOM” results from one player's uncertainty.
The second example I gave just there is a “non-action-y” question, in that it will likely result in some lore rather than an action, but that's OK too — the Thief's answer will add nuance to the situation, and possibly prompt someone else to take action! And if nobody does, still, you can reveal an unwelcome truth or show signs of an approaching threat or whatever suits the danger/opportunity that the phenomenon represents.