It's obvious why he takes off his shirt all the time. Imagine if you have a smoking hot goddamn volcano erupting with lava and shit, and then you put a sweater on it. What would happen? The sweater would melt.
Same thing here. Only Hosaka is pretty fucking good at keeping his powerlevel down, and only takes off his shirt to prevent him from sweating too much (obviously holding down a powerlevel of that degree is going to take a lot of effort, which in Hosaka's case causes him to perspire).
Also, unlike an active volcano, I bet Hosaka has got pretty good control over when to ERUPT if you know what I mean.