guys i need some advice, what causes brake squeal ?

replaced all discs and pads with PAGID parts - huge squeals when driving at slow speed and during braking, in fact wobbling the steering made it squeal, so changed discs 3 times under warranty with pads stilll no better, on 4th attempt changed the disc manufacturer on front squeal has now finally gone at last but now squeals from the back brakes, took rear pads off to inspect and they were full of meat at the top but there was a visible reduction of meat the lower down the pad in fact at the base of the internal pads i was scrapping metal the pads were literally worn at a diagonal - how can this be when the caliper is pushing the pad flat against the disc ? this was the same on both sides of the rear brakes ? total loss how this could of happened, the pads are only 10 weeks old, had to replace them so put on ferodo pads to see if these do the same.
Rear brakes still slightly squealing as i drive and noise gets louder when i brake - any views guys, is it possible that after market pads and discs do not match the dimensions of Ford parts and so there is movement in the calipers ? i am at a total loss, rear alloys are warm to touch after a run so still not running freely, all calipers have been checked and cleaned 5 times now.
On the verge of scrapping the brakes and buying new discs and pads all round but ford parts but this will cost a bit let alone the small fortune i have paid out so far - fluid was changed 10 weeks ago as well - any views guys. thanks in advance.