ladydobz

Do what you can to pivot towards the sensibilities of an academic historian as soon as possible. Your UG GPA, your success rate in previous cycles, and the rankings of where you applied are not very useful to you.
What kind of history do you want to do? How might your research impact existing historiographical debates of your specific field, your area/time period, and the overall profession? How do you see your career as an academic unfolding over the next thirty years? <==Talking about these kinds of questions is likely going to draw more initial interest than your backstory from academics.

Another possibility, just looking at your acceptances so far, is that they didn't actually think you would accept the offer and wanted to be able to extend it to someone else--congrats on Michigan and UCSD, amazing programs! 

This doesn't necessarily mean that people will come off the waitlist. 
Programs make offers knowing there will be a yield. For instance, if they have ten spots and they anticipate a 50% yield, they will make twenty offers, which means they will have their spots filled and no one will come from the waitlist. 
(Sometimes it happens that programs get a higher yield than they anticipated too and they have to shrink the next cohort). 
Of course, YMMV.