(I apologize for all the spoilers):
- Let’s start from S2, Ep1 A Scandal in Belgravia
Sherlock meets Irene Adler (a.k.a. The Woman) who is able to fake her own death by replacing her DNA samples.
At the end of the episode he helps her escape from an unavoidable death.
- Then we go to S2, Ep2 The Hounds of Baskerville
Sherlock learns about project H.O.U.N.D., a drug that makes you hallucinate and leaves you in a suggestiable state in which you imagine the things people say to you.
- And finally S2, Ep3 The Reichenbach Fall
When Sherlock realizes the next step in Moriarty’s plan is to kill him he asks Molly for help.
So… What’s my hypothesis?
What if Sherlock knew Moriarty wanted him to commit suicide.
When Moriarty shot himself he realizes the people has to see someone falling, someone that has to be identified as Sherlock, but not exactly him.
He then makes the call to Watson, telling him he’s going to jump and all has been a lie.
He managed to drug (with the project H.O.U.N.D.) Watson before calling him.
The body of Moriarty is thrown from the building, everyone thinks is Sherlock and he manages to escape.