How did Jim's mom die?

Director/writer Jon Hurwitz explains "why" (but not "how") Jim's mom died in this interview:

The first decision that we made literally was to kill off Jim’s mom. And we liked that for two reasons.

• One, we liked how that matured the franchise in a certain way, where it’s not just about sex and losing your virginity and the smaller things in life you see in the earlier films;

  When you get to be in your 30s, some serious things can happen to you, and we wanted to explore that with a relationship that you know and love.

  Jim and his father have always had this relationship where his dad is giving him advice and helping him along in life, and when you reach your 30s, you no longer see your parents as just “mom” and “dad”—you see them as actual, real people. You start giving them advice, and we loved that role reversal.
  
  

• The second thing we loved about it was the ability to, by making Jim’s dad single, unleash him into the world in a whole different kind of way.

  Instead of just staying inside the house, he’s able to go to the party and mix it up. We were able to pair Jim’s dad with Stifler’s mom; [...] It was too tempting not to have Jim’s dad and Stifler’s mom get together on screen and see what kind of magic we could have.

None at all. No mention is made of her cause of death in the film, and I've yet to dig up any official sources that state the reason for her demise.

I guess it was to create new storylines and no other reason. how she died is obviously not important to the writers or producers so the reason is not mentioned and left up to viewers imagination.