I think that is part of it. But this has happened at least three times now. So there does seem to be some underlying pattern. I wonder if there are large(ish) groups of related purchases.
Or maybe it is just what you are saying.
I dunno.
Up is good. How's that for a conclusion?
I found this quickly googling (not going to say it's 100% correct, but it's better than my speculation):

Amazon Sales Ranking and Calculator for Authors - Rob Eagar
Discover what the Amazon sales ranking really means and use a handy calculator to project your book sales in real-time from marketing expert, Rob Eagar.

It looks like the difference between #2,500 and #5,000 is .... 25 copies sold per day.
So random variance. In addition, it updates every hour, so it might just be someone ordered it right before you looked, and there were no orders when I looked.