My GUESS: Your tank vent is blocked (which may mean that your tank may not be empty). Either there are two thru-hulls/seacocks in your system, only one was closed...or, one of them doesn't close tightly and/or you may also have a leaky y-valve in the toilet discharge line. The pumpout was powerful enough to pull in water through the open seacock in its quest for air to replace tank contents as they were being pulled out. If I'm right, you're lucky that the pumpout could find a source of air...'cuz if it couldn't, any pumpout that powerful would have imploded your tank.
And btw, if all you did to prep for winterizing was pump out--you didn't thoroughly flush out the tank and rinse out the system, you weren't ready to winterize. Winterizing instructions are a sticky post at the beginning of the Plumbing and Sanitation forum (where you should have posted this).