The problem you are facing is one of the most common problems in the world of 3D printing. The causes for such formations are of a wide range and hence beyond scope. But the good news is, that there is a way to eliminate this problem altogether. The solution is to print a raft for your model.
In the world of 3D printers raft is a peel-able base which ensures that the model sticks to the print bed and prevent flaring of the base of the model. At the end of the print this addition layer can be peeled of easily. Therefore your prints would be safe from any patterns at the bottom, since they would be on the raft which could be peeled away.