I believe the cancel=true part keeps the focus with the text box and you can't go anywhere else on your form without exiting the text box first, so as you suspect the procedure runs again. You're stuck in a loop there until the requirements of the text box are satisfied.

Personally I wouldn't do that check until just before writing the info to the spreadsheet, then I'd check everything, one after the other, in the order things get entered, and set focus to anything not acceptable, displaying a message at that point, and exiting the write sub.