Normally flights at Pearson fly into and out of Pearson in an westerly direction parallel to the 401. Whenever that is not the case it is caused by wind or construction. There is no construction on the runways currently so it was likely wind. When the wind is strong from the south or south east (off the lake) the flights take off south over Mississauga and south Etobicoke.
Mystery solved...a wind thing. Cheers!
They usually pass a bit southeast of us. I don't remember hearing planes retract their gear before. I can't see them because of the trees all around us, and the first time I thought that one was preparing to land on the roof before I realised that it was actually ascending. Phew!