Was that in the Old Town square? I had the same weird feeling in Barcelona. The guide was so nice and smart for two hours, but the last stop was just this long, awkward pause by a cafe while he waited for tips. It put a real downer on the end. You could tell he was counting the cash as people handed it over. He also kept pointing to specific souvenir shops, saying they were the "best," which makes sense now if he was getting a cut. It kind of cheapens the whole experience when you realize you're part of a sales pitch.
So you figured out the free tour trick too? Totally works on tips. Did one in Prague, guide was great but spent the last ten minutes basically begging for cash. It's their whole paycheck. Some guides also make deals with shops or bars along the route. They'll swing by, say "get a discount here," and probably get a cut if you go in. Ends up feeling a bit salesy sometimes.