It seems like this is just a bit too close together for the colonists.
If you send me your save file, I could check it out and see if I can change the tree collision object.
I noticed this when growing apple trees were introduced into the game, and thought of the effect as a good game feature. Something that evolves as the trees mature - its not a bug, but something that changes as the apple trees grow and logical when I thought about it.
As a seedling it only takes a small space, but as it matures it takes over a larger area.
The simple work-around is to just make your paths a little wider (2 squares+) and it will solve any growing pains
Actually there are a number of small issues with building just a single width square path and should be avoided where you have regular traffic moving along it.