Why would my head gasket blow?

I have a 2006 bmw 325i and have replaced 4 of the 6 engine coils. About 2 weeks ago I noticed at start up that the car idled rough for about 3-5 seconds then was fine. No service engine light. I assumed that one of the older coils was going out. Just last week on start up, the car idled hard and stayed there. Now the engine light came on. I took it the my machanic and he ran a diagnostic test. He said it showed #3 was out. When he went to replace it he said nothing happened. He said that both the #3 %26amp; #4 were out-- although the diagnostic test didn't show this. He said that there was oil in the engine coil compartment and that both were %26quot;dead holes.%26quot; He said that i have a blown head gasket and he thinks a potentially crack head of which will cost me almost $10,000. But My car has never over heated and gets regular oil changes-- every 5000 miles. How could this be possible?
Why would my head gasket blow?
Time for a second opinion......
Why would my head gasket blow?
Definitely time for you to go somewhere else.
I would take it to a BMW service center and let them decided since they service BMW and only BMW.
There are various reasons for blown gaskets apart from the overheating one. A head-gasket is a precision component and when installed properly it will last the life of the engine.

Having said that any small flaws in either the cylinder-head or the gasket itself will shorten its life, as it has to deal with very high temperatures and pressures.

Allowing the engine to struggle at low revs in high gear for example can easily blow a gasket.

A blown gasket could cause the bad idling you mention but the diagnostics should pick this up as the pressure would most probably be lower on the affected cylinder.

It would be best for your peace-of-mind to take the car for a second diagnostic test.