Depending on how strictly you interpret the rules for Glyph of Warding, one answer here is that the glyph cannot be triggered by anything but a creature or the action of a creature. If the other five glyphs were programmed to activate when "a creature activates that other glyph," then it's clear that the creature is the target. But if the caster of the other 5 glyphs tried to program them to go off when another glyph was activated, that should have failed because the trigger didn't involve a creature.
The relevant text of the Glyph of Warding spell is this:
If the spell has a target, it targets the creature that triggered the glyph. If the spell affects an area, the area is centered on that creature. If the spell summons hostile creatures or creates harmful objects or traps, they appear as close as possible to the intruder and attack it.
Additionally, there's this part about the trigger:
You decide what triggers the glyph when you cast the spell. For glyphs inscribed on a surface, the most typical triggers include touching or standing on the glyph, removing another object covering the glyph, approaching within a certain distance of the glyph, or manipulating the object on which the glyph is inscribed. For glyphs inscribed within an object, the most common triggers include opening that object, approaching within a certain distance of the object, or seeing or reading the glyph.
Although it's not explicit, it's fairly clear from the text that the intention is that the trigger focus on a creature or its actions.
See also this answer about how Glyph of Warding works with Gentle Repose (which targets an object—a corpse—but not a creature).