The clot's probably just what normally happens when a blood vessel breaks - it clots off.
Most placentas have some areas of infarct - as Angie says, this is more worrisome if you have an unusual rate of infarcts for gestational age - and it's also common for there to be some areas of subchorionic hematoma. Small abruptions are common with preeclampsia.
It doesn't necessarily mean that you've got a clotting disorder, even an unknown clotting disorder. What's going on in preeclampsia is a conflict between the maternal and paternal genes expressed in the placenta over size of the fetus at delivery; if the argument between them can't be negotiated to an agreement, placentas start to fail. One possible maternal strategy to cut down on fetal growth rate is to detach some, but not all, of the placenta, so that it can supply fewer nutrients to the fetus, so the fetus grows more slowly and is more likely to fit out.
The underlying conditions that go along with preeclampsia are almost always conditions that mean you've got a hyperactive innate immune system, which means hypertension, diabetes, autoimmune conditions, and other conditions associated with baseline inflammation levels. The general problem seems to be that we've got a glass half full of inflammation already, and at some point in pregnancy, the cup runneth over.
Hope this helps! I'm finding myself prone to use too much jargon this week so holler if none of this makes sense.