We analyzed a series of solar energetic particle events in late April and early May of 1998, during which lunar surface potentials reached values as large as ~-4.5 kV (the largest recorded by Lunar Prospector). The two largest surface charging events during this time period correspond to energetic particle injections, when the electron flux between 50 keV and 5 MeV exceeded the proton flux over the same energy range. We searched the entire Lunar Prospector data set for other large negative surface charging events, and found that they occur almost exclusively during magnetotail crossings (when the Moon encounters the plasmasheet) and solar energetic particle events. Lunar surface charging (and its effect on the lunar dust environment) during inherently unpredictable space weather events represents a significant hazard for exploration.