Let us picture this scenario: I am standing next to two enemies, and attempt to move directly away from them both, meaning I leave a space in the reach of both. Both of them really want to hit me with their reaction attacks. The first one swings at me, and connects. Unfortunately (for somebody), that enemy also has the Sentinel feat (PHB, p. 169), which means:
When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.
I was just hit with an opportunity attack, and thus cannot move, so do I even give the other creature a chance for a reaction attack?
(This is of course just one example of such a situation; another is an opportunity attack which grapples me or drops me prone with insufficient movement left, leaving me unable to move.)