I agree with BitofFun And MorrisZap - it's usually not just because someone is posting for the first time and/or has name-changed.
I think some of the attitudes to this differ depending on how long you've been on MN too - if you're an old codger a seasoned regular of 7+ years then you'll have seen a lot of 'troll' threads and the way they develop. In the past many Mners have been very, very upset when they've posted sympathy, advice, detailed personal stories etc and then found out someone unpleasant was just stringing them along... and the whole OP was a fake situation. There have been some seriously nasty/ distressing ones too....
God knows why people do it, I agree, but they DO... sometimes it is attention-seeking, sometimes boredom, sometimes 'fun' that goes wrong, and yes, sometimes journos posing as newbies looking for easy copy, and new 'angles' for an article with a rapidly approaching deadline .
It's all in the lifecycle of a chat forum, isn't it... been here, seen that, and begin to spot the signs... And sometimes those signs may be mis-interpreted and people get offended.
Personally I don't think saying 'I'm suspicious' or 'Trip-Trap?' is that offensive, it's merely stating that you personally have some doubts and asking if others think the same?