What kind of animation/drawing style is used on K-On! Ura-on? K-On! Ura-on are bonus episodes from the anime K-On! and they used a different kind of animation or drawing style. 



This is the first time I've ever encountered such style (very different compared to the series, though you can easily tell the resemblance to the original) and was wondering for this style's name, if it have. Also, did other anime used this prior or after K-On! Ura-on was released? And, what is this style's history and background? Or is this just created to be unique or random reasons by the K-On! production?
As noted in the comments, this is actually the super deformed style. Wikipedia describes it as:


  Super deformed or SD is a specific style of Japanese caricature where
  characters are drawn in an exaggerated way, typically small and
  chubby, with stubby limbs and oversized heads, to make them resemble
  small children. This style forms an integral part of what Japanese and
  American anime fans refer to as chibi.


Some examples: