Not an expert, but I have two thoughts:

1. They don't have to wait until another miner finds a block, they can just wait "for some time" and then release their block. All that time gives them the edge for the next block.

2. My understanding is that if two different blocks are found concurrently for the same head, then the network waits for the next block to select which "new head" is accepted. I.e. when there are competing chains, the longer chain wins. So I could imagine that a strategy could be to wait until some other miner announces their block and release yours precisely at that time, hence creating two competing chains. But you presumably have an edge because you have already been mining for a while on top of your block.