Batter Interference or Obstruction on Bunt?

With no outs and the bases loaded, the suicide squeeze is on. However, the batter (B1) bunts a pop-up down the third base line. As the pitcher is standing on the third base foul line and about to catch the bunt pop up, the runner from third base (R3) collides with him, causing the pitcher to drop the pop-up. R3 advances to home and all other runners advance to their next base.
 
The offensive coach argues that the pitcher stepped directly into R3's base path and R3's contact was unintentional -- so there should be no penalty. The defensive coach thinks that both B1 and R3 should be called out.
 
What do the umpires do now?
 
Call
A runner is out when he hinders a fielder on his initial attempt to field a batted ball. If a runner interferes in such a way that prevents a double play, two outs can be declared (NF 8-4-2g).
 
In the situation below, R3 interfered with the pitcher's attempt to catch the bunt pop-up, so the ball is immediately dead and R3 is out. R3's actions prevented what was a likely easily catchable pop-up and then a double play on R3 -- so both R3 and B1 are declared out (NF casebook 8.4.2E).