Since the heyday of the United Farmers of Canada there are 75% fewer farms in Canada

While true, Ontario farms declined only by 1% from 1990 – when these rural areas by and large voted NDP – and 1995 – when they changed their ways. The declines were more like 10-15% every five years prior to that, as well as for a time after that, so this change happened during an anomalous period when there was effectively no decline in farms.

Living in a “rural” area no longer means you’re all that likely to own a farm

Essentially nobody in rural Northern Ontario owns a farm, but they have remained hardcore to the left. It is not that non-farming rural residents strictly hold different views either. What is interesting is that the same shift is observable at the exact same time in the rural southern prairies (like Ontario, the rural northern prairies also remained to the left), so it does seem that there is something, whatever it is, that targeted agricultural areas – although not necessarily farmers, as you point out – to bring on this change.