Yep, a reasonable question. There's a lot of conflating "seeking to explain", with "justification" when these types of incidents occur. It's completely acceptable to try and understand why something occurred, whilst also maintaining that it is entirely unjustifiable (which you are doing).
I asked the question about booze, because every time I've been to tannadice, it's been riddled with folk completely fucked, and it's the second time that throwing incidents have occurred. It's also correct to point out that had the pitch invasion not happened, the chair throwing wouldn't, Utd will have a degree of liability here and will probably get fined. Again, it's not an excuse, or justification, just evidence. As @Slim points out, they were teenage kids that were goading the Dons fans, it could have caused some serious damage. It takes a serious idiot to launch a chair at a crowd of people.