either the flow from your filter is too strong, your heater is malfunctioning and that side of the tank is too warm, or there's something in the decor that makes them feel safer on the other side of the tank.
Could just be they feel safer there. I used to have all my (imitation) plants at the back of the tank, and hardly saw most of the fish. Having now moved a couple of small plants near to the front, the fish swim much more freely throughout the tank.
My three guppies like to swim on the right side of my tank... They heater and filter on the left side. They do swim on the left side, but they like to go inbetween the plants on the right side.
I plan on getting some plants on the left side soon, I'm just a little tight on cash being a college student and all.