Were the PIK notes held by United or the Glazers themselves? I seem to recall it was the latter but possibly through their ultimate holding company Red Football. They refinanced them 5 years ago but didn't explain how, whereas if United had done that, they would have been some indication to show how. The original notes were secured against the Glazers' United shares and the likelihood is that whatever vehicle they used to pay them off is also secured against those shares. The whole thing appears to be a pack of cards which, Maxwell-like, is propped up by cash, shares and loans being shifted between all the companies.
Their main business is First Allied Corporation, which builds and rents out shopping malls. That's almost certainly very heavily mortgaged and they took out new finance just before the crash. They were very definitely struggling at about the time those PIK notes were refinanced, with some of the malls having gone into liquidation and others in the equivalent negative equity, with the rents barely covering the mortgage or even falling short. They negotiated loan that had an interest-only only option, which expired in 2010 so the payments shot up dramatically. It's quite possible that they renegotiated those loans at that time and, in doing so, raised extra cash to pay off the PIK notes.
The outlook for the sort of malls that FAC own isn't great. They don't have prestigious anchor stores that bring in big-spending customers. They're more the equivalent of somewhere like Stretford Arndale. Each one will have a mortgage and it's quite possible that, somewhere along the line, their United shares will be the security for some or all of that finance. So if ADUG or a related party were to buy those mortgages and foreclose, they could get hold of a significant number of the shares.