Flight routes

Hi , new to the forum. Maybe this question has been asked before but out of curiosity I'm wandering what determines the flight path of planes going over my city.
For example this week planes have been travelling over leeds uk mostly from Frankfurt and Amsterdam whereas when I payed attention in the new year most were travelling N/NW from Heathrow ,Paris and Zurich?
Is this a genera alteration of fight plans something that rotates on a regular basis or is there a science to it?

Thanks in advance