Just so i understand here - what we have seen over the last...

Just so i understand here - what we have seen over the last couple of days is likely a surge in the shorts buying back the stock. As i understand, UBS had purchased the stock and then onsold/lent it to third party 'clients' to short. So the process has been a large rally as these clients have bought back and then a large drop in price as UBS has sold back onto the market? is this correct? do get a double whammy effect here?

my question is: why would UBS do this - why can't the individual client just take short positions in the stock without UBS - why do they need UBS in the middle to take the initial position? how does UBS get paid for doing this - i imagine its just a fee, other than that what is the incentive? just liquidity and being able to offer clients a large cost effective position in the stock?