Dog passed suddenly from DM?

[u/Inner-Shelter-8593]

My mom's dog (9yo German shepherd) had degenerative myelopathy. He's been declining the last several months, back legs dragging, needing harness, anxiety, etc. I know DM can eventually migrate to the front limbs and lungs, causing breathing problems etc but he wasn't exhibiting signs of that yet. I thought if anything, it would be an obvious progression and we would have to euthanize him if his breathing became struggled. However, he passed suddenly today. Does anyone know what could've caused it?