1014 was a busy time for Londoners. The year before, Swegn Forkbeard, their unwelcome new king had died after only 5 months on the throne, and Aethelred wanted his crown back. But the Vikings were still in possession of the city, and they had other ideas. This time they were the ones defending London, and the attackers were Aethelred the Unready and his ally King Olaf of Norway.
Aethelred and Olaf were clever fellows and they protected their ships with thatched roofs pulled from buildings downstream. The bridge was packed with stout Vikings throwing everything they had onto the Saxon ships, who were busy tying ropes to the bridge piles. They rowed with all their might, taking advantage of the tide, and were able to pull out the supports, tearing the bridge down and everyone on it.  London threw open the gates and welcomed their old king back in, and the Northmen went away, only to return two years later with Canute and start all over again.
Many historians think this is where the nursery rhyme came from!