Many modern alternators seem to have a real problem with duty cycle and any sort of 'self protection.' The alternator on my POS 95 Olds handily failed after a battery discharged, (twice) which turned out to be the brake light switch sticking intermittently. The damn thing does not have 50K on the clock.
When you jump a rig using a running car, that alternator is suddenly subjected to the extreme charging load of a dead battery, and then you get in there and cause what amounts to a dead short on the output, IE cranking the starter.
There is absolutely nothing to limit current flow other than the impedance / resistance of the conjugate system you have just created, which, practically, is a dead short.
