That's only because of the reckless system Arsenal play, their CBs are exposed all the time and Koscielny is forced to make these sort of challenges in the box all the time. When he has Mertesacker as a partner as well who practically has no pace, he has to cover that up most times by being the cover defender and that usually means he's the one making last ditch tackles. As said before, due to the volume and amount of times they both get exposed, it's kind of inevitable. Koscielny would be much better here at CFC under Jose, he's better than Cahill in almost everything for me.