Simpson, the third-round leader, had gone 50 consecutive holes without a bogey but made three of them in a row starting at the par-5 12th.
"I felt really confident and then just got on a bad run there," Simpson said. "I'm probably going to learn something from it. Have to learn the hard way."
Simpson can now focus on family. He's skipping the British Open because his wife, Dowd, is due to give birth to the couple's second child within three weeks.
Joining Simpson at 11 under were Robert Castro (63), Kevin Na (65), Sean O'Hair (66) and Ken Duke (70). Duke was within a shot of the lead on the 14th hole before fading with a pair of double bogeys.
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