The potential causes are many, here is a sample list:
Faulty hardware (one bad network card anywhere on the network can create havoc; bad memory chips, motherboards, hard drives, hard drive controller, hub, network wiring)
Failure to use a UPS on every PC on the network - and a power blip occurs
Electrical/magnetic interference - for example wiring crossing or PC's near fluorescent lighting, copy machines or soft drink machines on the same circuit as any of your PC's; lightning storms, electrical surges/spikes of all kinds, boom boxes located too closely to network hardware
Importing data into Advanced Accounting data files via ODBC or third party software
Anything that causes a blue screen of death or that makes your PC "freeze" or that causes your PC to lose its network connection