Not condoning the behaviour (obviously) but I read this yesterday and wonder if it's related:
http://www.spiritofshankly.com/news/Information-for-supporters-_-Manchester-Utd-Away.html"LFC have assured us that they have contacted both Manchester United and GMP to raise these issues, and they have been told that again, supporters will not be allowed to use the toilets or other facilities after the final whistle unless under exceptional circumstances. However, no explanation of exceptional circumstances has been provided. They have informed us that supporters will instead be allowed toilet access up until five minutes before full time, and that there will be tannoy announcements giving supporters this information."
Apparently the toilets have been smashed up in the past. But not allowing people to use them seems a bit daft (and obviously doesn't stop them being smashed up either).