It is simple Microsoft and Apple are trying to get their paid 4.5Billion dollars from Google. It is a strategic mistake from Google side deciding not to be part of that team to buy the patents from Nortel.
MSFT/AAPL and the rest made a strategic decision to win those patents and destroy Android. They gave Google a choice:
A) Don't take patent - get opened for lawsuits
B) Take patent - lose trust because we are going to sue the shit out of Android OEMs
C) Buy all patent for an overinflated price
I just hope Google gets a team and invalidates all those patents, but I fear large patent price tag came for a reason.