is an outstanding short thread that discusses various compression aspects, focusing a lot on peak compression. You're experiencing normal compression, long term compression from encountering the thermal limits. They just can't dissipate the heat you're giving them.
Like Bill said, you're just trying to get more out of your speakers than they can take.
A by product of the current flow in the voice coil is heat. Thermally, your drivers (and crossover network) can only handle so much before the speaker's parameters change. The crossover points can change, etc. When one component in say a three way, enters compression first, then spectrally the output changes.
Simplified, compression is when a driver's output level ceases being proportional to it's input level. This causes audible changes. The prudent enthusiast, upon first hearing
such changes, stops increasing drive level, and backs off 'til the output is clean.