When AO-51 enters daylight the solar arrays start charging the batteries. This can be observed in the telemetry as a sudden increase of the current from the solar arrays. This time; however, a sudden drop in the current was observed shortly after the current started to increase, see the graph below.
The time and place where this unexpected drop occurred suggested that the satellite might have experienced the solar eclipse that was about to happen over the southern Pacific. It was Masa san, JN1GKZ, who noticed this and reported it on the AMSAT BB where after Mark, N8MH sent out the telemetry graph shown above.
A quick simulation using the Celestia space simulator confirmed that this was indeed the case. The video below shows how it may have looked like from the satellite.
You can try it for yourself. Celestia is available as free download from http://shatters.net/celestia - it's a great space simulator for demonstration and educational purpose. You will have to add the data for the satellite. You can use my data to begin with, download from here: http://files.oz9aec.net/video/SolarEclipse/ao51.zip
Unpack the ao51.zip file to the "extras" folder of Celestia, start the program and find AO-51 (type ENTER, AO-51, ENTER, then press "g" to go to the satellite). Enjoy!