It means exactly what I said -- We noticed that suddenly the crowds changed. We were seeing fewer families and instead were seeing roving gangs of teenagers that were obviously locals. They definitely weren't anywhere near as polite as the crowds we saw from the morning through early afternoon. Some of them were definitely up to no good ie: pickpocketing & such, some seemed a bit unsavory.

At one point my girlfriend started holding onto her purse and I agreed and moved my wallet to my front pocket, not that you can easily retrieve it from my back pocket.

Seeing the change in the crowd led me to believe that there might be some sort of afternoon/evening discount which might be for locals.