The bump starting thing is because of unburnt fuel as mentioned. The fuel will wreck the catalyst - partly why diesels have DPF's that belch black smoke. The ECU thinks it should be fuelling but there's no spark to ignite it.
Regarding the ECU thing - only a guess but:
A lot of alternators these days are controlled by the ECU so that it's managing the output of the alternator and the load on it. Putting a second car on the end of it potentially adds an unknown quantity wanting a big whack of power. Another possibility off the top of my head, is that when there's the initial required power surge down the jump leads to turn the starter, it will lower the voltage on the donor car's ECU - to compensate it then draws more current than it should. (Slightly similar idea - in old houses where the room lights dip if you turn on something needing a lot of power like a hoover). It momentarily takes a hell of a lot more voltage/current to start a motor from stationary then it does to run it at a constant speed.
