Let us say you stake and and the current block is #999. Then one of your unspent inputs stake and produce block #1000. Then someone else mines block #1001 and confirms the block you generated and your reward is safe..

Another scenario: Current block is #999. You stake and produce #1000, which is propogated into the network.
But at the same time (roughly) a guy named Charlie stakes or mines block #1000 as well (this happens because your block #1000 is not propogated to his node yet).
Now, depending on who creates block #1001, it can either build upon your block or the block mined by Charlie. And if block #1001 builds on Charlies block #1000, he gets his reward and your block gets orphaned leaving you with no reward.

This was written in a hurry, but I hope it helps.