Basically it all started when Lorenzo said in an interview that Pedrosa was not a nice guy, that he was distant with the rest of the riders and not friendly.
Then he complained about the weight advantage Pedrosa has, and then when both made contact in one race (can't remember which one, but in the contact Lorenzo crashed and damaged ended up with a twisted exhaust although he still went on and won).
Lorenzo said it was Pedrosa's fault because he closed the door on him.
At this point Pedrosa's team boss Alberto Puig exploded and told Lorenzo to shut up and win something before calling himself a championship contender, he also said Lorenzo has a crasher and the he was responsible for the incident with Dani and a similiar one with DeAngelis a few races earlier.
And that's pretty much it.