It's not the Glazers who are buying them. It's United. If you issue 1m shares and then buy-back 100k, that means you only have 900k in issue. If your company was worth £9m, then each of those 1m shares should be worth £9 in theory but with 900k shares, each should then be worth £10 so a buy-back is often used to drive the price up.
But it's usually done by companies that are sitting on piles of cash that they have no other worthwhile way of using, so as to increase the return to shareholders, or to fight off a takeover bid. Neither of those applies to them so my conjecture is that they're using the shares as collateral for other borrowings, maybe in their property company, and the price has fallen so low that the banks are saying those shares aren't adequate as collateral.