Apparently, Kapil got drunk and belittled Sunil and also physically assaulted him.

Mumbai: Here's some good news for all Dr. Mashoor Gulati fans as Sunil Grover is still part of 'The Kapil Sharma Show'.
Sources from the famous comedy show have confirmed that Sunil as well as his co-stars Chandan Prabhakar and Ali Asgar, who had boycotted the show after the fight on the flight with Kapil, are shooting for the next episode.
For the unversed, Kapil and Sunil reportedly had a fight as they were flying down with their team to Mumbai from Australia after a show.

While returning from Melbourne and Sydney after performing a stage show Kapil hit co-star Sunil Grover on an Air India flight.The incident took place when Sunil was on his seat and Kapil suddenly walked up to him and started hurling abuses. In no time, he held Sunil by his collar and raised his hand on Sunil. The actor, who plays Dr. Mashoor Gulati on the hit show didn't respond to Kapil's abuses and beating and stayed mum. The entire team soon came to Sunil's rescue and took Kapil away.
Apparently, Kapil got drunk and belittled Sunil and also physically assaulted him.
It's been a while now that Grover, Chandan, and Ali have not shot for the show.