1. Why did the PM resign?

Prachanda is one of the cleverest player of Nepalese Politics. He hits the target when most suitable comes around no matter where he needs to bow. Based on his nine months tenure, he (and whole maoists) realized that they did nothing other than further deterioting the  situation. They were looking for this unstable situation to leave the job so that they could get a great slogan "नागरिक सर्बोच्चता कायम गरी सु-शासन कायम गर्न देशी बिदेशी प्रतिकृयाबादी हरुले अबरोध श्रीजना गरेको " forever.

Now Prachanda and company will be more violent because they are "hurt tiger" after getting some fleshes after knowing the government taste.

2. What is the main cause behind all this turmoil in Nepal?

I say "Maoist Army". Prachanda and central secreteriate members know thatunless they give the "highly anticipated" posts for their PLA of cantonment, that can be the reason for their ultimate loss.