Sounds deliberate to me: Tell the Police now. Ask they or the Council to tell, not ask , the landowner to cut the field now - its an unacceptable health and fire hazard, and there are indications/possible deliberate fire starting.
Can the path be temporarily closed to the public, resident access only? Same as they do with road works so the legal provisions already exist.
Depending upon how many homes back onto this path a residents group may get some interest (or self-interest!) - a standard invite letter pointing out the current risks posted up a few times along the path, a common Whatsapp group they can join and post comments about any sus person seen on the path, local power/water etc happenings. etc. Offers of surplus produce, making fire brooms, etc etc.
The fact that there is one, or appears to be one, can scare off strangers or rein in errant neighbours/kids. Besides all the social good and getting to know your neighbours it can generate.
We have one for our road. It works quite well despite there being a distinct middle/working class social division/difference.