That sounds like a stress issue. How much decoration and what other fish are left in the tank? Where are the decorations located? (clumped together or spread apart)
Try changing the decorations around, add something new, and see if that doesn't do the trick. Also, watch the temp. If they are clinging to the heater, they may be looking for warmer temps. If the water isn't circulating properly in the tank, one spot near the heater will remain warmer than the rest of the tank. They may be craving the heat, and this, also, may have a lot to do with stress levels.