Kernel Problems randomly restarting MacMini Late 2012
If your issue appeared with 10.10.3, I am 98% certain it is a bug in OS X Yosemite, introduced with the 10.10.3 update and related to Intel HD 4000 GPUs. Welcome aboard. The kernel panics are totally erratic, some users have also graphics glitches. The only common feature is having a Intel HD 4000 GPU.

On MacBook Pros with dual GPU you can avoid the problem by disabling Automatic Graphics Switching (System Preferences > Energy Saver). 

Please contribute to the thread in the apple discussion forum and file a bug report (with link to the thread).