Why are my bathroom floor tiles lifting and popping up?

This is rarely wear and tear. Three usual causes: 1) the tiles were not bedded fully so there are hollow voids under them (tap across the floor — a hollow drummy sound versus a solid one tells you a lot, same as when tiles sound hollow underfoot); 2) no movement gap, so heat expansion makes them push against each other and tent up; 3) in a bathroom specifically, water getting under badly-sealed tiles and breaking the bond.

A couple of tiles can be lifted and re-bedded. But if the floor is lifting in more than one spot, that points at the prep or the waterproofing underneath — which is the landlord's problem, not yours, so document it. Get a tiling specialist to look before more let go.