Suppose a large pool finds a block with very little or no bonus reward, the pool can old this block and try
to find another – with better bonus -  block.
when the pool operator  will see another block announced, he could try to announce his "bad" block hoping
for it to catch the other block.
Now, there might be more than one  smart-ass like this, so each time 3-4 blocks will be announced together
forking the blockchain.