You're definitely on to something here. I can get it to crash with your second alias but not every time. Usually it happens the second time but sometimes it needs more. Even when /crash_mirc doesn't cause a crash, it leaves mIRC in an erroneous state. For example, after one run of the alias, the Editor forgot its last window size and font and opened in the default state. Various other oddities occurred as well.
Tested on Win7 64bit.