Woodstock was a festival that was in danger of being shut down for every second it stayed open. The overcrowding made it impossible to police, food and water on the site almost completely ran out, hygiene facilities were nowhere to be found and the local Governor viewed the site as a disaster area. It wasn't even clear that some acts could be helicoptered into the site until the day of their performance. And then there was the rigging.
You've probably seen rigging like the yellow structure in the picture at lots of festivals and major gigs. It's usually used to support speakers, lighting rigs, video equipment or whatever is required of it. At this festival, it was mostly used to support people who just wanted to get a better view. Unsurprisingly, it collapsed. What was surprising, though, is that it didn't kill anybody on or underneath it in the process.