My explanation is that the human mind is something like a coalition of different sub-agents, many of which are more like animals or insects than rational agents. In any given context, they will pull the overall strategy in different directions. The overall result is an agent with context dependent preferences, i.e. irrational behavior.

Congratulations, you just reinvented [a portion of] PCT. ;-)

[Clarification: PCT models the mind as a massive array of simple control circuits that act to correct errors in isolated perceptions, with consciousness acting as a conflict-resolver to manage things when two controllers send conflicting commands to the same sub-controller. At a fairly high level, a controller might be responsible for a complex value: like correcting hits to self-esteem, or compensating for failings in one's aesthetic appreciation of one's work. Such high-level controllers would thus appear somewhat anthropomorphically agent-like, despite simply being something that detects a discrepancy between a target and an actual value, and sets subgoals in an attempt to rectify the detected discrepancy. Anything that we consider of value potentially has an independent "agent" (simple controller) responsible for it in this way, but the hierarchy of control does not necessarily correspond to how we would abstractly prefer to rank our values -- which is where the potential for irrationaity and other failings lies.]