I think the leasing out and in makes sense myself. The aircraft going to Canada for the winter are A321s which are larger than the A320S being leased in.
Europe needs fewer seats in the winter months. I also expect the leased in A320s are not that expensive to operate and can be returned whenever. The 7 year
deal with Transat is lucrative because it keeps aircraft and crews busy in the winter with costs covered plus a bit of profit.