I live just south west of Jarrell and it is not uncommon for our stronger storms to move in odd directions. Some of the worst storms I’ve experienced came out of the northeast or northwest and moved south as part of a front or isolated MCS. I may be wrong but it seems like I remember that this tornado confirmed a theory of some kind about the overall system type being able to create a tornado.So, the parent supercell that eventually produced the Jarrell tornado formed right along a boundary, likely driven by a series of gravity waves that were coming in from a collapsing thunderstorm complex well to the northeast. This combination of factors, along with absolutely ridiculous instability, caused the supercell to sort of propagate along the boundary in the direction of the gravity waves (roughly southwest) rather than moving as you'd expect. It's a very odd and fluky series of events, but you see it happen occasionally under similar conditions, especially in central Texas.
You can see this play out pretty clearly in the visible imagery. Here I've marked the cold front (blue line), dryline (yellow line) and propagating gravity waves (orange lines). The supercell explodes basically as soon as the gravity waves begin to intersect the boundary, and then the whole thing just sort of "unzips." I included a bit more detail in my blog post if you're interested.