Because the thing they are about die, or the forum has bad leadership and lacks new attractions.
Look at Distant Horizon, a forum about ATLA. ATLA ended and the forum dropped from like 10,000 active members to less than 100 in a few moths.
But also look at PE2K, a forum that used to be very popular and where the administrator *used to* leave for months at a time, unannounced. Refused to have any other administrators or assistant admins. While on top of that, some of the forum placement and thread placement makes no sense and the only real updates to the forum that have occured were the adding of 4th generation forums. The forum hasn't seen decent new skin at least a year and has no good attractions, such as blogging or donation abilities.