Removing 2 treasurers at a time means there was no redundancy left if they both wouldn't cooperate.
The multisig address is a 3-of-5 address, with theymos being one of the three.
A new address was created with the public keys of the 5 new treasurers (which is equivalent to theymos, the two treasurers that are remaining, and the two new treasurers). The transaction out of the old address would have been into the new address.
None of the treasurers would necessarily have known in advance that any of the treasurers were being relieved of their duties, or if they could deduct that one or more treasurers were being fired, which of the treasurers were being fired until theymos released the public keys of each of the treasurers.