I would expect more people (especially bands of Spanish students) around Easter - April 1 to 5 in particular in 2010.
A lot of these youngsters (we met them both on the Camino Francés and on the Via de la Plata) were quite unprepared to walk, took the bus a lot, tried to party in the dorm where people wanted to sleep, and basically treated the whole thing as an excuse to get away from their parents and drink wine, while having the opportunity to stay in cheap dorms.
According to Spanish friends, this is most common along the last 100 km of the route, but we met plenty of them in other placers, too. I also heard (but have no personal experience) that there was a tendency towards lightfingeredness regarding others' belongings.
Whether they're allowed to cause overcrowding will depend on the hospitalleros, I expect.