R134a isn’t a very high pressure refrigerant. Maybe 100- 125 psig at ambient. Cylinder would be designed for around 200 psig or higher.
Doesn't take much carbon steel to contain that pressure.
Propane cylinders ( cooking gas) are about 175-200 psig at ambient and not in the sun. Design pressure is probably around 250 - 300 psig.
my guess would be external corrosion on weld seam ( picture seems to show some corrosion ) plus getting dropped on something pointy.