When I see an old fossil get dug up, I'd like to know why. If it's Community that did it, well at least there's an explanation. If some user did it, I often have to trowl through the page to find some miniscule edit, or a comment to a bottom-feeding answer, and sometimes I can't see the bump at all. Seeing old questions get plopped back on the stack for no apparent reason is thoroughly annoying.
It seems like it would be a good idea to have a function that people can look at to instantly see what the last bump-worthy action was. A description beside/rollovered the bump time ("edited 2m ago", "answered 7m ago", "commented 33s ago") seems like the easiest idea; on the question page itself maybe putting something similar just under the title (just throwing out ideas here). Noting whether the bump was deleted (e.g. Question is marked as active, but nothing changed, not bumped by Community) would also be helpful to know.
While this would really help for old questions, it would theoretically help for any question, especially long ones.