The only difference from my usual re-stringings is I went for medium strings instead of an aimless approach before. I may have used medium but don't know. I've only tried the low E so far.

Why is this happening and how can I solve this?

Thanks for any help.
I had to read this a few times before I was able to get what you were talking about. They are called bridge pins and they are lifting because you are restringing your guitar wrong. The ball end needs to be bent forward so that it hooks under the bridge plate of the guitar guitar. In a perfect world you shouldn't even need pins to hold the string in place. There is a tutorial and stringing an acoustic on www.frets.com I'll let you search the site to get to the right place.
Not taking any online orders.