Other than electrical elements which inherently cause phase shift (and enables passive x'overs to work), there are: room reflections and delays to waveforms introduced by A/D and D/A converters, microphone placement, wiring losses, RF/EMI, processors, mixers; anything that comes in contact with a signal has the potential of screwing with the timing relationship.
A good reference with more info than you'll know what to do with:
"Acoustical Engineering" -Harry F. Olson, 1991.
A good reference with more info than you'll know what to do with:
"Acoustical Engineering" -Harry F. Olson, 1991.