As there's a higher playercount, there's a large increase in tower activity and mob PKing. There's just more people to openly PK all over the map.
For sparring, queues are longer and the server is laggier. This leads to less and longer spars for each individual player.
For BKers, there are a limited number of baddies that are spawned at any given time. A higher playercount means more people competing for these baddies.

That's why summer is beneficial to PKing, whereas the other nine months are better towards sparring and BKing. I don't think it's a problem, it's just the way the game works due to the nature of those stats.