Winter, it was shameful but it was also different times back then. There was no social media (except what passed for gossip back then in a drunken haze in the pub) so all fans had to go on was what the very biased mainstream media crap spouted. They all knew that Fergus spelt trouble to their beloved Rangers and so they systematically tried to destroy McCann's reputation on an almost daily basis. They published or broadcast negative stories about him or the club trying to turn the fans against him. If there was nothing negative then they stirred things up with player's agents to see if they could invent something.
Possibly the only real alternative was fanzines like Not The View, which were not really geared towards siding with the Board either. So after years of being bombarded with such negativity, some people who wrongly believed that Fergus was indeed tight-fisted and holding us back by not throwing £millions to mercenaries such as the 3 Amigos, thought it right that they boo Fergus.
It is not something I would ever have done but there was a wee bit of understanding as to why they did it. I'm sure 99%+ of those that did boo that day will feel ashamed about it now.