What happened here?

[Cry havoc]

I've been playing these games for over 10 years and have just experienced my biggest loss in that time. The manner and context of it has left me genuinely shocked, and it was so far outside the realms of expectation that I'm struggling to understand it. Hopefully someone here can offer an explanation for why things went the way they did.
 
Full Disclosure: super casual player, managing transfers and personnel but using a Knap tactic: Tactic. Essentially a wind things up and see how they go approach.

Heading in to the game I was on an 8 match winning streak, including a penalties win away to United in the Carabao Cup.

Manchester United were 1 place above me at 2nd in the table, also doing well results wise

Team cohesion was low following a squad revamp, but wasn't having a huge impact on results/performances. Team morale was high, although Fabian Schar was sulking over not playing enough and this led to a team meeting with 3 other supporting players that did not go well. Personal experience in recent years of the series has been that even unhappy fringe players can have disproportionate impact on results, so this may have been a significant contributor. 

Below is how we fared in the Carabao Cup game, it looked like a competitive performance.

....and this is the match that forced me here.





3-0 down inside 10 minutes, all 10 goals scored by United without a shot on target in response. It was similar to watching a team 2 or more players down against a significantly better side, or playing a striker in goal.

I've had big losses before and was prepared for another here, even taking good form in to account, but not like this. The best explanation I can think of is a revenge drive, combined with a stronger league determination versus my morale blip. Even then, it's a very hard result to take, Any ideas what happened here?