Charging issue?

Yesterday I met up with some mates about 20ks from my place, turned off the bike and when I went to start it moments later it felt as though the battery had discharged. The display eventually died and push starting didn't do the trick. We ended up getting a battery from my mates gsxr and touched the terminals in series which cranked the bike right away.

After that she would crank fine? I checked the voltage this morning and was fine @ 12.9v. When it wasn't starting I also tried using the decomp lever to no avail. Bike is a 08 SM510R. Any idea what would cause this? Prior to the ride I had it hooked up to the ibeat for a tune but made sure I didn't have the bike on for prolonged period. It was only that one occasion where it wouldn't start. Bike is only ridden on weekends.