When I flexed my wrist backwards, the crystal on my titanium Skagen cracked!

This sounds rather unusual, as flexing the wrist can pull the pins out of the lugs but should not damage the crystal.

If it is still under warranty, then take it to a local dealer for watch.

I suspect that you have a rigid strap and this puts a strain on the watch. I gave up using leather straps 30 years ago because of continual troubles like yours (a flexible bracelet will prevent this) and have never had any similar problems ever since .

If you have to pay for a new crystal, then investing in an expanding braceletshould be considered.