What is Thumbing?

I went to Makarov.com and downloaded the Analysis Target. When I tried it my shots tended to hit in the C region which is notated as thumbing. Hmm. Googling brought up this PDF file, which says that thumbing is "applying too much, or inconsistent pressure with the thumb." This PDF says "Thumbing (Squeezing Thumb) or, Too Much Trigger Finger". Yet another PDF offers multiple causes. I could be snatching, anticipating, lacking followthrough, pulling, heeling or thumbing. Woh. I'll have to ask an experienced shooter to critique my technique.