On the whole airlines do not own airplanes, they are leased from major US and UK Banks (HSBC, RBS to name two UK banks who specialise in this) or from the likes of IFLC and other lease specialist.
When an airline goes into administration, if you are owed money you will want to keep hold of the most valuable asset of the airline in order to ensure that you get your money back off the administrator as quick as possible. The XL aircraft have all been impounded by airport operators and will not be released back to the administrator and subsequently the leasing company until they have recieved what they are owned from the administrator. Having the companies largest and most valuable asset will ensure you get your money back ASAP.
This is why XL's bankruptcy was not announced until 2am in the morning, the airline wanted to ensure all its aircraft had left SFB so that they would not be impounded (according to the Sunday Times, one a/c was chased down the taxiway by police at SFB, after CC told airport authorities the airline was going bankrupt as soon as the flight was in the air). This is also why one of the XL flights from SFB to MAN was diverted to CDG, the airplane operating the route was flown by XL Airways France, who had not filled for bankruptcy, but they didn't want to land at MAN and find their aircraft was impounded.
Upon airport authorities being paid their monies, the aircraft will be released back to the leasing company. It is then up to the leasing company to find a new airline to operate the aircraft. Sometimes this means the aircraft is sent to the desert for a couple of months, while someone is found or (as will be the case with the 737-900's XL just recieved) they will be snapped up straight away and have a quick refurb at a maintenance base before entering service.
As a leasing company it can be a real pain to have your aircraft impounded especially as the airline that has gone bankrupt probably owes you an awful lot of money as well!
