it could be the last bit of the tank settling in after the transfer, especially if you stirred the sand up and didnt rinse it when you moved it over. it also could be the depth of the sand, if it is between 1" and 4" - the so called no man's land for sand bed depth (too shallow to function properly as a dsb, but too deep to keep clean). i have a very deep sand bed in my tank and stir only the top half inch or so every couple of months. when i ran a shallow sand bed (< 1" ) in my other tank, i would stir problem areas every few weeks before doing water changes.