I had the same thought. By default, it sorts by "Start Date." However, almost every tournament starts when it is filled and not at a specific date. So in practice, if you change the sort drop down between "Registered Players" and "Start Date" it gives identical results. They are redundent because of how people use the tournament functionality. I think "% registered" would be more useful to show how close a tournament is to starting, since almost every tournament begins when it reaches a certain number of players.
One thing to consider is while users may want to join a tournament that is about to begin, it makes hosting a tournament more challening, since you need a large number of registrants to be signed up in order for your tournament to be visible when users browse the Upcoming Tournaments. Maybe everything would go more smoothly if by default it ordered tournaments by how recently the tournament was created and give users the option to sort by % registered if they want to join a tournament about to start. That way, you don't need to send so many invites to get a tournament full enough that it shows up for users who are browsing.
Sorting by "Max Players" seems a little odd as an inclusion. I guess some people may want to join a tournament with 100 players, but in general I think users may care more about something like number of concurrent games than the total number of players in the tournament. So there are more useful criteria that could be added to the sort drop-down.
I find that the tournaments that are listed on the earliest pages in the long list of open tournaments are the most likely to be joined by someone browsing the list. But the algorithm for listing order (sorted by start date) seems to be by the number of current entries. The problem is that an 8 person tournament is much closer to starting (or could be) with 3 of 8 entries vs. a 3 of a 100 person tournament. Therefore I would suggest that it would be more accurate (and fair) to order the list by the fraction of the tournament filled rather than the only the number of registered players. Thus an 8 person tournament with 4 registered players should be listed in the region with other tournaments that are half way filled.