it happened to my post either, it was deleted without any info. suddenly, it was decrease. maybe because i posted in off-topic
for reply/comments.. there's two possibilities.
1st. your comments/reply has been deleted (if this happen than you should get some inbox from forum moderator)
2nd. main thread that you are commented has been remove/deleted.
but if that happen to thread you created, then off-topic is one of so many reason