Hypos can have many triggers. Exercise, cold weather, hot weather, revision, nerves, stress, for me my pancreas could secrete some insulin randomly and of course there's giving too much insulin for food amongst many other causes.
But some triggers are little weird aren't they? A friend of mine's diabetic brother goes hypo everytime he sets foot in H&M. Yeah, H&M the shop sends him low. How weird?! They've given up shopping in there!