Orphan blocks happen when two blocks are mined at the same time. The block that gets a further block added to it will become the legitimate chain, and the other block becomes an orphan block.

It's the reason why Satochi said to wait for 6 confirmations to be absolutely sure your transactions confirm with near zero risk of being orphaned.

Having a chain recorg after one block happens. Having a chain recorg after 6 blocks is improbable.