According to grandfather’s sibling[0], it may be that the crossing onto the other land became a subject of an adverse possession. Apparently in such a case you’re supposed to have a clear notice[1] on the property in question if you don’t want to lose it.

[0] https://news.ycombinator.com/item?id=11494002

[1] https://en.wikipedia.org/wiki/Adverse_possession#/media/File...