Lots of things cause amps to run hot, or get hotter then normal
Running the amp at a lower impedance then its rated for. (if the amp is 2ohm stable, and you play it at 1 ohm it will run much hotter then it would if it was wired at 2 ohms)
A bad ground - could be a loose crimped terminal, loosely bolted to the car, a bad ground spot, wire to small.
A bad power wire - mostly the same as above, loose crimped terminal, not fully tight to the battery, to small of wire.
Electrical system is too weak for the power the amp wants, so the amp is being starved trying to make power it can not make because the voltage/current is not there.
Having your Gain / Bass Boost set too high and are causing clipping and or distortion, which is making the amp work harder.
It could be a cheaper amp, and when it was put into the heat sink there was little to no thermal paste or pads used to transfer the heat from the mosfets to the heat sink.
Could just straight out be a poorly designed heat sink that does not dissipate the heat good or as fast as it should.
So yea. lots of things can cause the amps to run hot, or over heat, and if the amp is from a good stand up company with quality products, chances are its usually caused by user error.