can time machine start preparing for another backup before the prior one ends?

I have a drive that I use for time machine backup that I had in a safety deposit box for a couple of months. I am trying to run time machine using that drive to add the last 2 months of updates to the drive, and understandably it will be a long run because a lot has chenged. So, I clicked on back up now, and it finally completely the "preparing" step and had some 58 gigs to process. So, it started running for several hours and seemed to have processed 3 of the 58 gigs. Then, all or a sudden it's doing a "preparing" step again right then. So I'm wonderiing why it didn't seem to completer the first, large update before it started "preparing" for the next one.. Is it possible that time machine can start preparing for another backup before the prior one ends? If not, what might trigger this....