A few times it's when things are going predictably well, and another contract is reduced to just grinding it out as opposed to trying something different. As others noted, sometimes it's been when the unit is too large (that was around hitting a Level IV at the end of the Jihad).
Most of the time it's when a new release comes out that solve a few annoying issues, and it's easier to start fresh than transfer things over. Not so much at this point, as I've modified the availability XMLs so that takes a bit of diff-checking to merge in added units.