It was quite common at the time to boo John Barnes. It started because of the England crowd booing him at Wembley for his perceived underperformnces for England (He didn't play that badly for England, more that he was just outstanding for Liverpool).
i think a lot of came about when digger said he didnt get the whole ' st george, england pride' thing the supporters do.
i remember being at an england game against,i think , norway,when all the england fans boo d him.as a liverpool supporter,it really pissed me off,but yeah,it was very common then.