Term for the reverse of hijacking a meeting?

The phrase bait and switch is often used to describe a process in which an attractive product is offered to induce buyers, and once they engage with the salesperson, they are diverted to a different, more expensive product, usually being told that the original is not available or otherwise undesirable.

The term is also applied to politics and other dealings in which an innocuous proposal is deceptively introduced, only to be substantially changed (as was the original intention) to something much more controversial.

You could say that another meeting attendant co-opted your meeting:

We were supposed to talk about the new project's budget, but then Bob co-opted the meeting to rant about support issues with our main vendor.

The second defintion is:

to use or take control of (something) for your own purposes

Source: http://www.merriam-webster.com/dictionary/co-opt

Another phrase you could use, which I've heard more than a few times, is going off the rails, as in:

Our meeting was going well until Bob sent it off the rails by spending the next hour complaing about support issues.

You could also use the word dominated, as in:

Steve tried to run a meeting about the budget, but Bob dominated the meeting by constantly discussing support issues.