Tropheus Bembas Rubbing Against The Gravel?

[mikeplatt101]

Hi all, I purchased agroup of 22 Bembas a couple of weeks ago and I've started to notice a few of them rubbing against the gravel. I know this can be a sign of disease but they are not showing any other signs and they look perfectly healthy otherwise. The water quality , PH Hardness is perfect so is this normal?