When blocks come in super close together a miner will accept the block **header** of the latest and start mining before actually downloading the entire block and verifying all the data.

However, by only using the header, they can’t put any transactions in the block, because they don’t know which ones to remove without checking the last one. Sometimes they find a valid hash in that small period of time where they don’t yet know what they can actually put into the block… and thus, it stays empty.