Sorry- I ramble. Because the letter c is high up the aloha bet, chat topics were always at the top of the list, so other threads got overlooked. By calling it _chat, it sent the chat topics to the bottom in active convos so you had to scroll past the other topics to get to chat, which kept the other topics a bit busier as people would be more likely to see them and post.
It's to do with alphabetical sorting, so anything with an underscore goes after the normal A-Z.
Similarly, if you look at the A-Z list of topics you'll see that Forces' Sweethearts is right at the top - above the A's - because it's actually listed as "Forces. Presumably the " makes it superior to A, IYSWIM?
It's to do with how computers sort things. Every character - letters, digits, punctuation - is identified by a different number. For historical reasons (that I could bore you insensible about if required but that is explained here) it ended up that the order of characters was:
some punctuation characters one of which is " Numerical digits 0-9 a few more punctuation characters Capital Letters in alphabetical order even more punctuation one of which is _ lower-case letters in alphabetical order and a few more bits of punctuation
When a computer does a simple sort it uses this order. Hence why something beginning with double quotes goes first and something starting with an underscore ends up after everything starting with a capital letter.