Six again IS as Padge says, a penalty on the run. It's more punishing than a penalty kick as it does not give the defence time to reposition, and enbles the attacking side to regain momentum.
Of course, there is a simple way to reduce the number of six-agains.
Re: injuries. Without real data on number, types, circumstances; without data on player age, position, experience years, new or recurrence of injury and more, much comment is surely subjective.
Just like this: the SL newbie clubs are more affected because they are now involved in higher - intensity games, with more players not used to that, and maybe not used to the higher level of training that may involve. I don't have data to support that, so it's speculation.