I just got it to crash again. This time I simply double-clicked to select a group of units all of the same type and then single-clicked on the first unit in the group. Then the game crashed.
So I've done some experimenting. I've tryed it on several games using different races. Here are my findings:
1) Go to the Army setup screen in a province that contains multiple units of the same type.
2) Either Double-click on a unit to select all of that type in the row or Shift-click on a two units to select all the units in between.
3) Clicking once more on either the unit which was double-clicked or the first unit of the Shift-click will cause the game to crash.
4) Clicking on any other unit in the selected group simply removes that unit from the selected group as normal.
I've reported this bug - though I experienced it when I had selected multiple units with 'w', and then attempted to examine the initial guy. But I think your description goes a little bit deeper.
I don't think PC users have been able to replicate this behaviour - it may be Mac-specific.
I tend to do lots of extra clicking there as well, especially playing LA Ermor with the tendency of undead to reanimate in such a disorganized manor. At least its a general bug and not just my computer. So, it could possibly get fixed, eventually.