Why Would A Patient Have Pericardial Bleeding Post Stent Placement?

Why would a patient have pericardial bleeding post stent placement? An 85yr old female patient was scheduled to have five cardiac stents placed but were only successful in placing two of the five during the surgery. Surgery was then scheduled for placement of the three remaining stents the following day. The patient within the first postoperative hour exhibited hypotension and further testing revealed pericardial bleeding. What could have caused this and what might the prognosis be?