iPhonedownlow.com

This is the best explanation I could find for why some iPhones get stuck in an endless reboot. (I could be incorrect on some of these points)

Here’s the problem – Springboard is watched by a watchdog process. The watchdog process monitors Springboard, and if it doesn’t respond at least once every couple of minutes or so, it is killed and forcibly restarted. If this happens approximately 4 times, the watchdog will reboot the device. I’m pretty sure this is to ensure that a hung /unresponsive Springboard.app will reboot–> fixing itself instead of freezing the touchscreen until a force reboot is manually performed.

When installing an application the Springboard generates an Application Map with the location of the various icons and their screen location. Apparently, after Quickpwn is finished and the iPhone reboots, the Application Map must be completely rebuilt. So, of course, the more Apps you have installed, the longer it takes to rebuild the Map.

Therefore, if you have a sufficiently large number of installed Applications that causes a regeneration of the Application Map to take longer than 2 minutes then it becomes impossible for Springboard to complete the task before it is force quit by the watchdog. The phone will be rebooted by the watchdog process after several minutes, run out of time and reboot the device indefinitely.

FIX YOUR IPHONE BY FOLLOWING THESE STEPS!!