Juro, my misspent years in SQA finally avail me!
Threads come unwound under 2 circumstances (i) someone replies to a thread from a previous board; the reply lands on the current board, but follow-ups do not thread; and (ii) someone changes the thread title by adding a comment or date.
Once a thread has stopped holding together as a consistent grouping it never recovers. The message for those who care [and until there's a new board -- although, Mark, I think I've beta-tested two or three new boards here over the last coupld of years so I'm not holding my breath] is to (i) start new threads in the current month when responding to a prior month's issue, and (ii) either don't mess with the title of a thread or start a new thread if you feel compelled to alter the old one's header.