Here's a little cigar 101 for you. When a cigar starts to dry out the wrapper dries out first, when that happens like leather it shrinks. When that cigar is placed in a humidor and the humidty is reintroduced to that cigar the filler and binder swell and as a result the wrapper often splits.
There are other things can cause wrappers to split, poor construction is often the culprit.
And of course ensure you have a stable humidity level in your humidor. 68-74% is a pretty wide range, but it seems to work great for me.