Geograph - photograph every grid square

As we walked along the remote River Humber embankment at Cherry Cob Sands, we passed about 500m of the river sloping side of it on fire or mostly having been on fire. The top of the embankment was normal and so was the marshland vegetation below it. There was nobody looking after it - only a farmer and his assistants harvesting a field about 500m away, so unlikely to have been them. I don't know what the purpose of burning this vegetation would be anyway - it didn't seem to be doing any harm to anyone! Fortunately, the fire seems to have been in a controlled manner, although worryingly though, the fire had apparently, in two separate places, been right next to discarded propane cylinders that had previously been washed up on the marshland some time ago! Anybody any ideas as to why there was a fire here... or had it just been a carelessly discarded cigarette that had caused it?