The underside of the JK root crown: these tiny white hairlike roots are fast-growing and brittle. Disturbance of these greatly assists its spread...and triggers its response of quickly forming viable nodal material from which a complete new plant is effectively "cloned".
These root systems are extensive and difficult to identify with an untrained eye.
Under-estimation(as arguably apparent at this site) of JK's extreme and aggressive habit, is certainly a chief cause of fresh infestations: scattering and mere disturbance generates much JK response as it seeks to extend its rhizomes quickly down a path of least resistance, before it explores and exploits structural weakness in more difficult terrain.
A field under crop...as here at Standish...is a perfect combination of vectors for its growth: mechanical spread of material could be extensive in its consequences if field margins are trimmed and fresh knotweed growth tackled incoherently.
The field soil is regularly under plough, thus a perfect growing medium for JK above and below ground.