McShane, who left the Stadium of Light after a dreadful two years in August last year, suggested the Boro midfielder's reaction to his challenge led to the mass fracas.
Twenty players rushed over to play a part in the melee, in which McShane looked like he had thrown a punch in the direction of Joe Bennett as the shoving spilled over, splitting an advertising hoarding.
Thomson, a ?2.5m buy from Rangers last summer, felt it was like being back in an Old Firm encounter rather than a Championship fixture between Middlesbrough and Barnsley.
It was comical to say the least. These things happen, said Thomson. I will watch it back and I am sure we will all think it shouldn't have happened.
I have had a few of those (melees) in the tunnels before in Scotland. It's part and parcel up there. I wasn't expecting it here. There were no bad tackles about. I was annoyed that he came through the back of me and I reacted, then it exploded.
McShane felt the whole furore was unnecessary, claiming the intervention of nearly every other player on the pitch was what convinced referee Chris Sarginson to issue him with the red card.
I felt Thomson kicked me and it was a bit sneaky, so I reacted the way I did, said the Republic of Ireland international. I didn't throw punches or anything.
It was the melee afterwards that maybe got me sent off. If the lads had just let me be, it may have been two yellows for me and Thomson.
I probably over-reacted. I am sure he has sent me off for over-aggressive behaviour. My sending off changed the game a bit. It happens in football and I think referees should be a bit more lenient at times.
McShane's dismissal proved the turning point in the game, with Middlesbrough suddenly given the encouragement they needed to cancel out Marlon Harewood's first-half opener.
Although Thomson may have left the pitch five minutes after the scuffles through tiredness on his first start since New Year's Day, he was satisfied to see Scott McDonald rise to head in the equaliser ten minutes later.
It might be still mathematically possible for Middlesbrough to drop into League One, but with a sevenmatch undefeated run behind them there is an everimproving confidence within the group ahead of tomorrow's trip to Burnley and Thomson can feel it.
It's been a dull place to be this season, said Thomson. I have been injured but I could still see the players come in with glum faces, and that's not usual.
The atmosphere you could cut with a knife during difficult times, but thankfully we have steadied the ship. We are trying to play the way the manager wants. We have a group who want to listen and that has shown through.
When Thomson moved south of the border he did so with the aim of achieving promotion to the Premier League in his first year.
Instead he has been left frustrated on the sidelines, nursing himself back to full fitness as his team-mates battled to avoid dropping into League One. Now fully fit, he could be forgiven for wishing the season would last a little longer.
It's a Catch 22. I am desperate to keep going and get fit because I want to keep in decent shape, he said. But we will be starting to think about next season soon. The manager has brought in stability. You can't take anything away from him for that.
I don't think it is fair to say that he is getting the best out of us where the previous manager didn't. It's different.
The manager here is loved by the fans, he has an aura around him here and it shows.
Whether he will be deemed fit enough to figure at Burnley tomorrow, with Julio Arca and Matthew Bates expected to return, rests with manager Tony Mowbray.
But Thomson said: I feel fine. I expected a bit of cramp watching Britain's Got Talent and the Barcelona v Real Madrid game on Saturday night, but I feel alright.
Middlesbrough could have Jonathan Franks and Luke Williams back in the squad for the trip to Turf Moor after returning to training after ankle injuries, while striker Leroy Lita should be back in contention within a week after making good progress from a hamstring tear.
Source: Northern Echo
Source: Northern Echo