Can someone elaborate on this? I didn't understand anything besides "transaction DDoS". Is it what happened?
It's pretty much very close to "transaction DDoS". There was a lot of invalid transactions that flooded Bitcoin network which caused some bugs for pools and exchanges.
They weren't exactly invalid, they were changed in a way. Here:
https://en.bitcoin.it/wiki/Transaction_Malleability you can read more about it.
Actually a very good article. Leaves no questions behind.
someone tried PPLNS mining on 50BTC?
I did. It was "no block" day so didn't learn much about their distribution.
And how can I learn pool's hashrate?
Pool's API should give answer to this. Also