In particular, it seems the EC is worried about the future of MySQL. To date, some of Oracle's biggest competition has come from MySQL, which is one of the most popular database suites around the world. With millions of servers worldwide relying on MySQL, the EC is concerned about how Oracle will treat the software once it belongs to them. If they decide to intervene, it could result in a months-long setback, which will likely result in further harm to Sun.
At the beginning of this week, Sun disappointed investors with news that they suffered a $147 million loss for the quarter - they are surely eager to get themselves into Oracle's hands. Sun stock continues to fall, though the deal is currently on-schedule. It remains to be seen if the EC will initiate a probe into the deal.