Pardes:
XL's problems was nothing new. There was no surprise as far as Sunwing having a contingency plan. I am guessing Sunwing decided it would be unwise to put all eggs in the same basket considering the current economic climate in Europe and the state of the aviation industry in general, and was therefore already in talks with other European tour operators long before XL shut down.
As for "diversifying", Sunwing is to the best of my knowledge not altering its course or venturing into new and uncharted territories as far as flying goes. They have simply alligned themselves with new partners in terms of aircraft and crewing. This will ensure enough A/C and crew is in place this winter and allow allocating of redundant A/C and crew during the off season.