Rain is out, grip is gone.
Firstly, while some of the drivers in this video are obviously frequent visitors of the Nürburgring, not all of them are pros. Most of these drivers get to enjoy a day on the "Green Hell" during the Touristenfahrten sessions, or "public driving" sessions. In case you didn't know, the Nürburgring Nordschleife is considered to be a public toll road by the German government. This means that for about £30, any street-legal vehicle in safe condition can take a lap of the famed course. The vast majority of the Nürburgring Nordschleife has no speed limit during these public sessions, so yes, you can go as fast as you want. And that speed plus a wet track can catch even the most seasoned driver off guard.
Gallery: See what happens when amateurs drive on a wet Nürburgring
In dry conditions,the Nürburgring Nordschleife is already an extremely challenging circuit, requiring immense concentration and some degree of memorisation for each of the 160 turns over the course of 12.9 miles. It's a fast track that isn't exactly forgiving, and the weather is known to change at a whim, and it's not unlikely to have a quarter of the circuit drenched in rain, while the rest is sunny and dry. There have been a number of improvements done in regards to safety, such as new barriers and better stations for marshals and emergency vehicles, but it's still a long way to go to be considered as safe as other circuits. During open driving days, those who pay the toll for access are also required to pay for any damages done to the track. Any fees corresponding to any towing services are also shouldered by the involved parties. Also, failing to report an accident with another vehicle or track surface is considered illegal. This is all part of the rules and regulations which aim to ensure a safe experience for all visitors to the track.
Thankfully, not many barriers or cars were harmed in the video, and most of these drivers just experienced a brief embarrassing moment. Oh, and the number of BMWs going sideways is pretty staggering; it doesn't help that most of them are trying to drift on purpose, which is also illegal on the Nürburgring Nordschleife.