I think they are "consolidating" all the coins they have mined in to one large UTXO so that when they pay the miners in the pool the transaction is one large UTXO to many, many outputs instead of the alternative without consolidation: many mined blocks of about 25 BTC to many, many outputs to all the miners. The alternative to consolidating the mined blocks into one large UTXO would cost more to send because it would be a much larger transaction.
Just a guess.
Thanks, that's what I vaguely suspected too, but I wasn't sure.