Since priority is a global state and a task can appear in multiple projects, rearranging the ordering on one project's workboard can change its global priority, and push it down (or up) on another project's workboard.
The ordering of cards on a workboard shouldn't have to be directly tied to the task's priority.
Yeah, this seemed like a good idea but was a bad call on our part. I'm planning to make the default rule "not ordered by anything / stay where I put it" and possibly let you reorder by priority, but we'll likely do better UI cues if we do pursue this (see https://secure.phabricator.com/M10/10/) and provide some sort of "put it in the right place in this column" interaction versus "put it in this place specifically" (some discussion somewhere that I'm not immediately able to dig up). I'm going to merge this into T5675, which covers supporting a "stay where I put it" rule and making it the default.