This is a risk of any mri machine that uses helium to cool a magnet. Why are only these machines being recalled when if you block the exhaust of any mri scanner it will blow up?
This is just speculation, but maybe there is a secondary rupture disk that vents the helium inside the room when the primary path (the exhaust to the outside) is blocked. And maybe that secondary rupture disk is too strong, so another random part of the helium containment fails, blowing up the machine and possibly injuring the patient. But please note again that this is pure speculation.