From my own personal knowledge of Eton, I would say that some of the leggit footage is from an old leggit. That apart, boys protesting in the form of a leggit is an old and valued tradition amongst the boys, and not the least bit intimidating. Noisy, yes, and it often causes passers-by to stop and watch, but not intimidating.
The beagle pack(s) were an essential part of Eton and boys who were not keen on rugby etc. could spend time exercising the beagles as one of many alternatives. The packs would be seen going on long runs in and around the grounds every afternoon, boys herding them while on their bikes. When the beagles had puppies, it was a notable highlight every time. Boys were very fond of the beagles and any headmaster discontinuing the dogs will be unpopular.
On top of that, the current Head is not liked for many reasons too numerous to mention in detail here. A previous, very approachable deputy head left because of the new Head. Once gone, his role was carved up and the school run as a "business" rather than the very pastoral touch it had before. New hierarchies were created with staff departments which had so many managers that those nearer the top were unreachable, not the very personal, family feel the school enjoyed previously. It is good that the boys have the means to make their feelings known about the way the school is run, they are the ones who live there after all.
Oh and I suspect that the pp 'nolanscrack' is not a troll but an insider. "Nolan's Crack" is an Eton name for a cut-through walkway which runs alongside some of the boarding houses, only an Eton person would know this. (And no, I am not, and never have been, an Eton boy).