Most reputable Christian leaders, even those with very large ministries (Franklin Graham, Max Lucado, et. al) have accountability partners who they can talk with and run ideas past, and these people can tell them if they're 'going off the reservation' scripturally. They put themselves under authority of fellow believers to help them stay the proper course in their walk with Christ. Many of them also have pastors and churches that they are a part of who have authority over them.
Most nutjobs or cult leaders, like Camping, don't have anyone in authority over themselves, setting themselves up as the highest authority. In their minds, the only one they answer to is "God".
(Note: I put "God" in quotes because a lot of times their ideas are so half-baked and strange that the true God had absolutely nothing to do with it, even though they give Him 'credit' for it.)
Camping did have resources for accountability, and chose to ignore them. He was teaching at Alameda Community Church, of the Christian Reformed Church denomination, where he was asked to stop teaching on May 1988. My guess is that the other elders of the church had previously approached him with their concerns (a Biblical approach BTW). Camping chose to walk out in a huff. His claim that the Holy Spirit left the church on 21May1988 and no one would be saved after that may coincide with the date that he was to stop teaching.