What causes the tractor protection valve to close automatically?

AAMVA CDL Manual, Section 6.2.2, p. 6-5

This automatic safety feature is what keeps a trailer breakaway or a major air leak from draining the tractor's own brakes along with the trailer's. Knowing a drop into the 20-45 psi range triggers the shutoff helps you recognize the warning signs, falling air pressure and a popped-out knob, before you lose control instead of being caught off guard.