For some reason this is has been ingrained into his game over the years. Not excusing it because I hate it myself, but I think it started after his jaw was broken by an Aberdeen player in the Scottish Cup final of 2017. He used to get clattered and battered by thugs in Scotland on the regs and referees used to let it happen.The whole "not enough bad guys" narrative is so annoying. If players show character by standing up for themselves and their team-mates or call out others for their bullshit (like Tierney did there), I like to see that.
Pathetic stuff like Tierney's "head injury" (Evans did the same thing in the first half) on the other hand needs to get out of the game and be punished with suspensions.
So since then Tierney has had a tendency to make a meal of nothing challenges in the way you've seen a few times at Arsenal.