Why does my crema clump up?

How can I solve this clumpy crema?
What usually cuases this problem? Is it badly roasted coffee? Not properly rested coffee? Or bad technique?
My machine is laSpaziale S2; extracted 38g from 18.5 in 31secs.
Image is taken 40 seconds after I pulled the shot.

Initially I would think that has to do with your pressure/bars on the machine itself. If you are not able to see the amount of pressure/bars while pulling a shot I would look into modifying your setup with a gauge in some way.