Why is Bittylicious sending out replaceable transactions?

Bittylicious is using a new feature of Bitcoin Core called replace by fee.

Technically, this means that transactions sent out by Bittylicious can be replaced by another transaction before it gets confirmed. The reality is that this allows a new feature to send another transactions, overwriting this one, but with a higher fee.

In the rare instances that a transactions takes a long time to confirm, it means that there may be the option to bump up the fee to get it to confirm more quickly. Bitcoin Core supports this only in certain instances though.

As with all transactions, once they are confirmed, they can pretty much be considered definite. When this happens, transactions are final and replacements cannot occur.

This is detailed in the Bitcoin Core FAQ.