Interesting! Make sure there is dura rock behind the tile (not drywall) and it's sealed properly. Dura rock is like concrete. You will have to remove one of the cracked grout/ tiles to confirm in most cases.
If you find that there is drywall behind the tile the bathroom shower tile it will have to be removed and the shower area redone correctly. Drywall absorbs/ wicks water and will cause expansion and contraction.
Another less expensive issue could be they did not use the correct type of grout or the less expanse type during the flip.
Either case it would be best to ask a qualified contractor to take a close look at your shower and perform a home inspection might help too.
For more information:
Read "What is the best backing material for shower walls?" a Q&A answered by Polly Osborne.