90% of the time it is this or temperature. ^^^
The rest of the time hardware isn't getting along (system usage, memory speed and availability, rogue processes) or you're pushing the card too hard. Back in my infancy when I used the GUI-heavy variant for mining I'd just put in settings I found on the wiki for cards and be baffled that I'd get system resets and the like. Pay careful attention to stats from cgminer - just made a modification to frequency or voltage and have a high HW or R rate? Tweak down and retry.
If you have windows, try something like GPU Caps Viewer for monitoring temp/cycles. If you have MSI, try using something like afterburner (may have name wrong, it's the MSI monitoring app) to see the usage graphs to trace the drops. For example, one MSI card I had tweaked to the 9's but my cooling wasn't adequate so it'd throttle for temp and eventually crash the drivers. Removed the plastic cover and did some aftermarket hack cooling and the card screams hashing now.