I'm a rugby player and its a few things. Firstly university rugby isn't like club rugby the majority of the time and tbh has a lot of stuff that club rugby really looks down on, such as excessive initiations and overly sexualised or dangerous activities. Universities and the RFU are cutting down on that heavily though, as they should be. My clubs 'initiation' is you stand on a chair after your first match and down a pint, which is pretty standard for most clubs.

Secondly it's a physical sport, its effectively a fight, and one where you need to have your team mates back. It requires much stronger bonds than in most other sports because you are going out there and hurting yours & other people every game, every tackle, every maul, breakdown, scrum etc. As part of this you do just get more comfortable around each other and have a different kind of relationship.

Thirdly it requires a decent degree of confidence to play rugby because its physically very tough, this typically attracts more confident people that will do more unconventional things. Not everyone is and equally not everyone is expected to get naked.

I'll admit I find it funny when people get naked and do stupid but innocent stuff. So long as it doesn't involve any unwitting bystanders its a bit of silliness in an all too serious world.