Lock Monitor

Lock Monitor thread detects deadlock. It does that by inspecting processes ecery 5 seconds to determine if there are any deadlock cycles. When it finds one, it automatically chooses one thread as the deadlock victim. It then rolls back the
victims transaction, cancel its query and returns error 1205.

The Lock Monitor chooses the deadlock victim by choosing the least expensive transaction to roll back.
You can overide this by using SET DEADLOCK_PRIORITY to low