Sub, when you bridge a two channel amp, the amp actually reads half the ohms. So when you bridge at 4 ohms, the amp actually sees 2 ohms. Most 2 channel amps are only 2-ohm stable. So if you hooked up a 2 ohm load to a bridged amp that is only 2 ohm stable, the amp would read 1 ohm and not be able to handle it.
Amps shut off normally when too low of an impendence occurs. If they dont have this feature and you dont have enough air flow, expect a fire, and or a fried amp. Seen it happen early 90's with my bros car . Most amps will auto shutoff now tho, unless you get a generic amp. but hey most of them shutoff auto too.