Also this whole thread could just be a thread about the Houthis. There isn't a rigged or re purposed weapon they haven't loved. Using S-75 SAMs as surface-to-surface ballistic missiles, using AA-8 air to air IR missiles as surface-to-air missiles, ripping the antiship missile launchers off Yemeni navy ships and converting them to mobile land based launchers, cutting two Scuds in half and welding the fuel tanks back together to get extended range, and this doesn't even begin to cover their improvised land fighting vehicles, remote-controlled suicide speedboats, or drones.
I think the dynamic behind it is this: a rough third of the Yemeni military defected to the Houthis. They had a lot of obsolete conventional weapons but knew they would never survive in the face of Saudi and UAE airpower, so they just stripped all the parts they could and got creative. I think it's the presence of lots of military hardware floating around + a sudden asymmetric threat that leads to a lot of these rigged up weapons.
That same dynamic is also why what might be the 20th century's biggest relative haul of captured non-standard equipment- North Vietnam's inheritance of huge quantities of US-built kit after the fall of Saigon- never resulted in any field-rigged battlewagons. They got enough A-37s, M113s, Hueys, and F-5s to just.....use them. Conventionally. Against threats they expected (the Khmer Rouge and China).