Beowulf was the last king of the Geats. Now that their warrior king is dead they remain vulnerable to attack from their neighbours. The warriors basically turned tail and ran leaving Beowulf to tango with the dragon by himself. Apparently the Swedes were just itching to attack now that Beowulf is dead. Wiglaf, being a bit of a downer, foresees the Geats will be attacked and their entire nation will be destroyed.