The explanation I got from CHP Commercial division, is that people were taking tractors from job to job, and leaving them running. The trailer bounces a little, the loader or backhoe lever jiggles, and now the FEL or BH is moving...
They claimed this had been the reason for bridge, overpass, and powerline damage. There aint no way a FEL is going to raise and hit an overpass unless the tractor is running... At least if it is chained, it shouldn't raise.
The regs should say "no running vehicles during transport"...