While I no longer have the 9530 (had it for a year, loved it, but upgraded to the 2), I can recall a time or two when this happened. It's not a big deal, more hassle than anything, but the device decided to do a random reboot just at the time you turned your phone on. I assume by "turn on" you mean, you hit the power button, the phone was really just asleep. It's possible your memory had reached a critically low level or, there was an app conflict. Keep an eye on it, if it keeps happening, you will have to look deeper.
