The location they rally around is random (annoying in of itself) but they do love collecting around traps. When a trap inevitably springs the convoy gets broken.
Traders have been known to chill on traps, and blame me for getting delimbed.
Most likely fixed in A17 (there's some related fixes and I can't get it to happen, so, "probably fixed".) Thanks for the report, and if it happens again after the patch, please let us know :)