As I understand evolutionary change it seems to stay realtively dormant for long periods and then it goes in spurts.
This is punctuated equilibrium.
It's not so much that evolution stays dormant, it's that there are few available spots to be taken up.
Think about it like this. In the early 1990s there weren't that many new businesses because, unless you invented a brand new product or service, there was already someone out there doing whatever it is you do. You, as a pizza guy, have to compete with established chains, etc.
However, when the Internet Boom came along, suddenly a vast new area of resources opened up. The number of new businesses went through the roof. Some were adaptations on existing businesses (Amazon.com) others were completely new (ivillage.com). Some succeeded massively, many many others failed.
It's much the same with evolution. The dinosaurs were doing well and mammals barely had a foothold. Then, whammo Asteroid, dinosaurs take a powder and mammals have access to the resources.
Suddenly, even a minor evolutionary change, gives Mammal A access to some untapped resource. Where as in the past, that resource would have been already being consumed by Dinosaur A.
This doesn't need to be trigger by disaster either. Say Animal B developes a way to consume plastic as food. Suddenly, dumps are a gold mine for this creature. We'd expect it to really take off and do well. We'd also expect that different versions of this creature would evolve. Perhaps one that digs very well. Or one that is better at eating plastic bottles. Or one that's better at finding plastic trashbags. Etc.
Hope all this makes sense, because WOW, I've really gone off topic.
Sorry, back to Bible Time