Why did London bridge fall down?

There have been a number of different London Bridges over the past 2000 years. In 46AD, the Romans built the first bridge across the Thames River; it was a simple wooden construction which was burnt down in 1014. The replacement bridge was destroyed by a storm in 1091, and the next bridge after that was destroyed again by fire in 1136. Another stone bridge served as the main bridge for centuries until, by the early 1800s, traffic congestion prompted the necessity for a new bridge. The bridge was designed by engineer John Rennie and finished in 1831. The design was superior, containing only five high arches, and constructed from strong Dartmoor granite. 70 years later, it was necessary to widen the bridge, a process which weakened the bridge's foundations to the point where it began sinking an inch every eight years. In 1968, it was auctioned and sold for $2,460,000 to Robert McCulloch who moved it to Havasu City, Arizona, where it was rebuilt brick by brick, and finally opened and dedicated on 10 October 1971.