Can I know what happened actually?
A thread which you've posted in was moved to the trash can.That's why you should pick the threads that you will reply on since not all topics are constructive and some are trashy so trashy things should be placed to where it belong i.e. trash cans.
Its going to affect my signature earnings!! 
Yes, luckily. A classic example of 'Crypto world spammer problems'.
Your point is legit and I hate spammers too, however I wouldn't blame someone just for being concerned about their activity rank because that is just as legit. Especially given how obscure it seems to be, i.e. not explained as clearly as it could be and sort of hidden in threads that are a little hard to find. A good professional forum would have Community Rules highlighted at the top so newcomers could get on board and up to speed right away.
As for trashy threads, you're perfectly right but again: if a thread has been allowed to get started at all then it must have been approved by a moderator. And if the title was so trashy and not constructive to belong in the bin, why did the moderator leave it instead of binning it straight away??? The way I see it is that no matter how trashy a thread might be, if it's there then people have the right to reply and their activity should be counted.
The truth is that the people in charge of this forum would rather have as much traffic as possible to show for, however trashy their threads, rather than put in an effort to filter quality at the source.
