[Ygg2]

There was a Quora by a Pixar employee (who did distance himself from Pixar, so not official statement) comment on that. I'll paste info here because Quora sucks:

   It's a wonderful example of how, given a sufficiently large dataset, 
   one can cherry-pick what one needs to construct almost any narrative.

   My daughter broke the time line, though, by pointing out
   that Heimlich has a cameo in Toy Story 2. I do want to 
   give the author props for inventing the idea of the 
   Monsters Inc. doors being time portals. It doesn't really 
   work, but it's witty.

   As in the case of Andy's father the real reason Pixar 
   films contain references to other Pixar films is rather 
   mundane. It's only because somebody thought it would be  
   funny and the director agreed. In the cases of things like 
   John Ratzenberger's voice and the Pizza Planet Truck, 
   it's  tradition and a challenge.

Source: http://www.quora.com/Pixar-Animation-Studios/What-do-Pixar-e...