It's just bored, sad people with nothing better to do it seems!
Because they have an irrational level of dislike for a particular housemate (Dexter this week) who usually happens to be extremely popular they think by starting lots of very similar threads with slightly different titles bashing the housemate and making often false or highly exaggerated and negative claims (Twitter/Facebook hates this housemate today, my favourite housemate is now my least favourite housemate, this housemate is gay, etc. etc.) that it will somehow convince fans of this housemate that they have backed a wrong-un and should jump ship now before they look foolish.
I personally find the idea that anyone with half a brain-cell would be swayed by such ridiculous idiocy hysterical and it just annoys people on the forum and blocks the forum up with loads of similarly laughable threads on exactly the same subject.
I think it makes the people who do it look like *****-tards!