It's pretty natural to want to figure someone out when they annoy us but here's the thing, they've probably just poked the sleeping bear. Who cares what makes them the way they are? We don't have to go over their history and assume we know anything about them. Even if we do get it right, they're not going to change for us.
Pro tip: Set some time aside and feel where the annoyance lands in your body. Does a memory come up? Remember your body is the storehouse of the answers.