Why is it somedays when I log on, older messages appear at the top of the board above newer ones, even when there has not been a reply?

sometimes between authorising posts several come in and bounce the thread to the top.
Then again often we deem the posts inappropriate or boring and don't add them, in which case the thread drops back down again.
And if our US or Aus boardkeepers are working, this can happen in the middle of the UK night.
We only put up a small percentage of posts.
We like to keep the quality high.

We actually took our board down, due to the impossible task , every morning, of removing adverts from a car loan company in the USA, that was automating their postings. Even when we tracked i.p`s, and deleted messages, we could never remove the top post they had left on the board, which appeared to be the tag that the system sending the messages would hook on to.
Even when the techie guy came to sort it out, he had no idea how they were doing it.
It became a worrying thing having an unmanned message board, mine was up about six years ago, and some of the things that would be left on it, were just awful, when you checked it.
My least favourite thread that kept appearing in various formats was "I went to see you play again last night, and you don`t know who I am.."
Yukky stuff that makes you`re skin crawl and "Play Misty For Me" springs to mind.
The bottom line is, it is a full time job keeping a board going.

It was a key decision when we started 10 years ago; likewise, when Paul started his Record of the Day (after my enforced absence).
It proved a very wise decision as these boards continued throughout my wrongful imprisonment with board keepers updating it regularly and posting my own thoughts on my behalf (via phone dictation).
Which drove some people (i.e. the Evening Standard) crazy!