:One of the theories, but hardly a proven fact. Read Idaho Falls for what I think is the definative public account. The operator who pulled the rod out too far, John Byrnes, was recently separated from his wife (crashing at friend's places for the last several nights) and had money problems. Byrnes was mostly at fault in the marital strife: he was a philanderer and drunk. 

 :Richard Legg's wife was a local Mormon girl, several months pregnant, and by all accounts hardly likely to be engaged in any kind of relationship with Byrnes. Most likely, they never met. Both the widows Legg and Byrnes have denied there were any extramarital affairs among them. 

 :The murder suicide rumor had swirled around the industry for years following the accident. It became a semi-official claim when a nuclear safety researcher repeated the rumor in a memo stressing the importance of making reactors operator-proof. Previous reactor safety guidelines always assumed the operators were in their right minds when at work, and instead concentrated on commies with grenade launchers. 

 :The most plausible reasoning I've read was Byrnes resented Legg because Legg was recently given a supervisory role. They had fought some months earlier at a bachelor's party (both men were drunk, and Byrnes had just fucked a whoreLegg probably taunted him about it). Byrnes had a temper, and had been known to throw shit around inside the reactor control room when frustrated. He was recently passed on the same supervisor job. The men were several hours behind in their night's task list. Legg probably said something to chide Byrnes (perhaps officiously restating the rule that the control rod mustn't be raised too far, or telling him to "excersize it" to prevent sticking), and Byrnes just had a "to hell with this" moment and raised the rod too far to piss everybody else off. Kaboom. 

 :There's no evidence anybody, least of all the poorly trained operators, knew that withdrawing the rod too far would lead to an explosion. They knew it could possibly melt down the core, but that wouldn't constitute a murder-suicide.