#1: The game recently went on sale.
The game's had more exposure and it's sale brought in people who might have previously been skeptical about purchasing it.
#2: More interesting servers have been popping up recently.
I don't know how much this can be stressed but we need to make and host more gamemodes. Even TheBlackParrot's pictionary brought in a boatload of people. When there's no interesting servers up, people scan the list and then move on. When there's interesting servers up, it tends to catch people's attention and have them get on servers more often.
#3: It's close to the holidays
Generally there's more activity around major holidays.
I knew about the first thing with the Steam sale, however in my 2-3 years of playing, I have never really noticed a significant player difference on during the holiday season. Definitely more, but nothing substantial. However, during the summer there are always lots of people on.
Also I wholeheartedly agree with you about others hosting game modes. I just think people should start hosting things which are fun and allow for the user to become involved, such as a simple DM. I may even start hosting a simple pushbrooming server soon, maybe it'll attract some users.