今天下雪！到底怎么回事？！

今天只是10月29号，

但是现在已经在下雪了。Don't add the 在, 在goes before actions(verbs) i.e. 我在看电视(I'm watching t.v.)  For this sentence, it isn't needed: 但是现已经在下雪了

怎么到底回事？

   秋天几乎不开始了 remove both "bu" and "le". The sentence you wrote is like saying "fall is not starting anymore" 了in this case would show a change in a situation, i.e 我不去了- I'm not going anymore(I was but there was a change of plans). 没 should be used here, since it indicates incomplete actions/events, i.e. 没吃-didn't eat. 没去 -didn't go. 了 cannot be placed here since 没 and 了 are completely opposite.了is for marking completed actions and 没 is for negation. Here's my correction: 秋天几乎没开始过

By the way I know! It was snowing heavily over here, too!

今天（竟然）下雪（了）！到底怎么回事？！

今天只是/才10月29号，但是现在已经在下雪了/就已经下雪了。怎么到底回事？

秋天几乎不开始了！还没开始。

fighting！

今天就下雪！到底怎么回事？！

I just want to add this word to your tittle,I feel it will make the readers more clear when see your tittle.