Cam Shaft Broken. What could be the cause?
 

I have a disco 2 2002(w/O secondary Air) 150k. My engine(new heads) stalled the other day . i tried to pullover only to hear an explosion noise and my oil all over the place. I looked under and the short block had a 3 inch hole in it right under cyl 6. I have had a loss of compression in cylinder 6 for a few weeks now and i thought it was bad piston rings. i started taking the engine part today and after removing the exhaust manifold gasket i saw that my Cam shaft is missing a piece and broken between Cyl 7 & 6. my rocker arm(driver side) had the rocker broken at cyl 7 and the 2 of the lifters on that cylinder we sunk into the engine. My question is what could of caused a cam shaft to break that way. i have not pulled the block out to see what kind of damage i have around cyl 6 where the hole is.

PS: i have replaced the lifters with refurbed ones that compressed very nicely when i put them in a few weeks ago but now when i took them out they were all locked as they if they are full of air, is that normal or something is wrong with lifters as well?
Sorry for the lengthy post, all your help is appreciated

Stuck lifter = bent pushrod = snapped cam?