According to Cydia developer angelXwind, it is caused by “unclean rebootsâ€. She explains:
My theory is that unclean shutdowns are, if not the cause, a contributing factor to BLoDs. I’m saying this because I’ve seen a lot of people get into BLoDs just by performing unclean shutdowns. What’s an unclean shutdown, you ask? An unclean shutdown is a device shutdown that does not properly unmount the HFSJ+ filesystem on iOS — this can be holding Lock and Home to force a reboot, or even a kernel panic can cause these.
So what exactly is the Boot Loop of Death (“BLODâ€)?
The Boot Loop of Death (which I will call “BLoD†in the context of this post) is not like the boot failure I described above. The boot failure above is perfectly normal, and the device will eventually successfully boot after a few tries, at most.
The BLoD is a peculiar thing. Actually, I — nor anyone else — really understands what the BLoD even is. Because it can get devices stuck in the boot process for literally hours, it appears to continue on forever.
[..] here’s what is a BLoD:
When you try to boot your device, the untether fails conseecutively for hours — or, perhaps forever. [..]
However, she goes on to clarify that getting stuck in reboot loop after installing an incompatible Substrate tweak is not a BLoD. If your device gets stuck in a reboot loop, you can address the issue by holding the Volume Up trick.
What should you do to get out of “Boot loop of death�
How can you avoid “Boot loop of death�
So here’s what you should do to avoid the mysterious “Boot loop of deathâ€. If you’re on iOS 9.0.x, and have already jailbroken your device using Pangu 9 jailbreak then avoid rebooting your device as much as possible. Respring is fine. It also means that you should not allow your device from running out of battery.
If you’re on iOS 8.4 or earlier, and have jailbroken your device, then avoid upgrading to iOS 9, and jailbreaking your device using Pangu Jailbreak until the root cause of the issue hasn’t been identified and fixed.
