CRC is a flavor of checksumming - error checking, so the root cause is whatever, on the technology you're seeing them on, could cause one or more bit errors in the traffic.
As mentioned, for Ethernet, it could be a duplex mismatch (though collisions are the more common indicator).
For serial WANs, it could be local noise (serial cable too near a sparky electric motor), a bad receiver in the serial interface, or something like network noise in the carrier network (bad contacts, bad termination, weak equipment interfaces).
For fiber it can be a loose connector, dirty connectors, a fractured fiber, a weak signal ...
SO, there's no one cause, not even a "typical" cause, even if you're talking about a specific transmission medium or technology (which you did not do).
The answer is "Whatever can cause one or more bit errors in the transmission" ... you have to look at the medium and the path, and decide what is the most likely place & reason, then begin your troubleshooting.
Good Luck
Scott
