Trek 4500 rear derailleur hanger bent again

Hi folks I had my bike serviced in November 2019 and one of the things that needed sorting was the derailleur hanger the guy tried to straighten it and it then had a bigger crack(not massively obvious,but a stress crack could be seen) in it so he replaced it,and bike was going great.
A couple of months after the service the chain seems to be slipping as you can be riding along and you can feel the chain slip on the teeth and it makes a right noise,it's so annoying after things running so well after all the gears being cleaned etc.
I've just taken it to another shop today and the guy says your hanger is bent he tried to bend it back but has made it worse(he's replacing it for free though),I'm just wondering if anyone knows why this would be happening again ?,I've never had it happen to a bike before,it wouldn't surprise me if it happens again down the line,the lad has ordered the part for me and he did say my chain is worn so he's going to replace that for me too.

I only use my bike to cycle to the shops and I as far as I'm aware it doesn't lean on anything when parked that would be causing the bend in the bracket which makes me think it's happening somehow when riding