The cause of a massive explosion last month in Fitchburg that leveled a home and scattered debris over a half-mile radius, severely damaging several other homes, may be tied to two mistakes made five years apart. Madison-based attorney Dan Rottier said the first oversight occurred in 2011 when his client, Cheryl Drive homeowner Brian Grittner, had his gas-powered dryer replaced with an electric one, and the dryer installer failed to cap the gas line during the job, Rottier told 27 News on Tuesday. The second mistake happened on Aug. 25, the day of the explosion, Rottier said, when tree trimmers working on Grittner’s property needed water and asked Grittner to open a water valve in the basement to allow for that.