In the 25th minute in a match dominated by Southgate's men, the Bulgaria number 3 went to ground and put in a hard tackle. The players argued with the defender about the hard tackle and he spoke calmly with them to make peace.
The same didn't happen in the stands. The fans took it out on the Manchester City player, who started to hear racist chants. They were in Bulgarian, which made many doubt if they were actually offensive in that regard.
But the referee realsied it and confirmed it, therefore he decided to stop the match and not restart it until the tannoy announced it. This, that was given in both Bulgarian and English, avised the fans that if they continued like that, the match would be abandoned.
The English national team were already considering walking off the park in case there were more racist attacks. This is a real intention of the players, who talked to the referee for a long time about the possibility of already walking off but preferred to give it a second chance.
It goes without saying that UEFA's rules clearly specify that, in these circumstances, the match can perfectly well be abandoned. Shortly afterwards, the game had to be stopped again because of complaints from Southgate, and fans who were behaving incorrectly left the stadium.