Nikon sued Sigma for patent infringement of their VR technology.
From what I know Canon started the idea of lens based stability. Now why Canon didn't sue Nikon if they think they stole the idea? was Canon the first one to apply patent?
Ideas are not patentable, Designs are. Nikon uses a different design than Canon, so there is no issue.
Apparently, Nikon feels that Sigma has used some of the design specifics covered in the Nikon patents.
It is a very complex subject, with lots of twists and turns. Usually, it gets settled out of court, but sometimes a little sabre rattling takes place first.
Since a lawsuit was filed, now Sigma gets to see Nikons evidence and claims, and may eventually decide one way or the other. likewise, Nikon gets to see Sigma's defense, and can better evaluate their chance of winning.