I'm not sure management even know they were terminating him until recently. They probably had a lot of internal discussion about it as well as trade calls around the league. From what has been leaked out about past trades, Dorion isn't a GM to play things close. He does a lot of homework and calls. Dorion had a presser on the 23rd and there were indeed some hints that changes were coming.
I know people don't like press interviews because they don't really say much other than cliche statements, but you can tell a lot by reading between the lines. For instance, Dorion mentioned:
1. Exploring all the options. I know that sounds like something every GM does, but he could also have said fifty other non-statements like "We like our team" or "We are looking for a couple young guys to step up". If you're looking at options, you're looking at trades, signings, buyouts, etc.. You don't like your team as constructed and will be doing something.
2. He mentioned trade calls and having to make sense long-term. He could have said other things but he specified ling-term a bunch. If he didn't care about age, etc.. and was just interested in improving the team, he would have said something like "A lot of quality players available and finding the right fit, etc.." or "It is still a seller's market.." If he's repeatedly mentioning long-term, if you read between the lines you get "People are trying to offload veterans but we aren't looking at older guys because we're rebuilding so we're waiting for a younger guy to come available."
I mean, there is only so many options out there.