I suppose it could be an extrapolation of the use of "plant" to mean a factory; perhaps each machine is in itself a "demolition factory". Or it could come from the fact that original end users of hired equipment may have been large "plants" and the name stuck?
I am, of course speculating and as they say, the US and the UK are "separated by a common language".

I am going to be honest here too, I have never heard of this either.

Now I have heard of "heavy plant" equipment, just describing equipment in a manufacturing plant. But never heard in terms using for describing excavators and the like. Maybe it's a UK term? :confused:

never heard of this