He's not biased towards anyone, he just likes picking a winner in a match and his commentary reflects the narrative he's trying to create. The problem more recently is that he doesn't just show this when discussing the game in general, but does so repeatedly when discussing specific incidents. I could understand Le Tissier going nuts at half-time about the disallowed goal, but Tyler was almost as bad on commentary and that's where I think the problem lies for a lot of people.
He went on and on about Gabbiadini's disallowed goal all game, which ended up contradicting his own statements. The goal, had it stood, would have changed the game, so to repeatedly state that Gabbiadini should have had a hat-trick was just stupid.
Everyone's initial reaction to the Stephen's challenge was that it was bad and worthy of a red. It was only after the third slow-motion replay from an angle no official would possibly have been able to view it from that it appeared to not be as bad as it first looked. Tyler then went on about United players surrounding the referee, completely ignoring that he only knew it wasn't a red thanks to a number of replays.
Later in the game he made a comment about Marriner being fair with his distribution of cards, then made a remark that basically said Lingard should have been sent off for a little trip in the middle of the pitch, when he'd completely ignored Romeu, who was also on a booking, elbowing Herrera. Instead he chose to say that United players were trying to get Romeu sent off unfairly.
After the game he made some weird comments about Pogba celebrating, calling him "PR Pogba".
He's gone bonkers enough over a number of United goals to prove that he doesn't have any real bias against us, but I think people are justified in calling him out for yesterday.