If you're consistently seeing the same pattern then the most likely culprits will either be your tamp or a clogged shower screen. Water is going to follow the path of least resistance, so if you're getting faster extraction at the back there is either less coffee there, it's less tightly packed (tamped), or there's an obstruction to the flow at the head (probably the shower screen) causing more water to flow to the back.

Considering the consistency, I'd wager it's a bad habit in tamping method. It tends to be easier to put more weight down closer to your body, meaning less pressure applied at the back, meaning water has less resistance flowing there. Something you might try to see if it makes a significant difference is to change the orientation of the portafilter while tamping. For instance, if you usually hold the handle straight out from the counter, you can try rotating it to the left, applying a light tamp, then going to the right and applying full pressure, or something like that. If you find that the pattern changes significantly when you make adjustments like that, then you know where the problem lies.