I've tried searching around on this and had a lot of trouble finding a concise answer.  It looks like if you send your troops on a long march to farm a resource tile, that tile appears as unoccupied to people viewing it in the navigator while troops are on the march.  They can then select to capture it and if they are closer or their troops faster, beat you to it and an attack can occur when your troops finally get there.

The problem with this is that it has political ramifications.. it can cause an unintended attack against a target that you had some form of alliance with and cause drama blow-back onto your clan.

This isn't so much a bug report as a feature request.. If the above behavior is as intended I would ask that the navigator window at least show when someone is marching towards a tile and how far out they are. So at least that form of tile baiting is intentional.

Even better, if you're farming with your shield down, and a target is unoccupied when you start your march but later becomes occupied. Your troops should hold and wait for the tile to clear or get an attack confirmation from the Jarl. it could be a setting applied at the start of the march, auto-attack, hold and wait, or auto-return .

I think it is too much to expect people to always throw up a shield while farming to avoid this problem.