I have spent many hours abord Lear 20 and 30 series aircraft. There is nothing wrong with them and they do glide. They are NOT idiot proof, and do need to be flown by a professional, trained crew.
I know of no mechanical or structural defects that would cause this. The situation had to involve crew incapacitation, probably passengers as well. The pressurization system on the aircraft is of 1960s vintage, but it is fine if properly maintained.
The biggest drawback with the early Lear series is that because of a relatively low street value, they generally wind-up in the hands of shoestring operators that try to cut every corner imaginable. That is a very bad combination.
I suspect you can get a Lear for pocket change when its engines have zero hours left before major overhaul.
This seems to be the case with the T-39/Sabreliner.