iPhone 5S - Vibrating for no reason when plugged on charger?

[AceC]

I let my battery die on my iPhone 5S and the ringer was also on. When I plugged it on the charger, it took a few minutes for it to come on, but when it had the white screen w/ the black apple, it vibrated, but I'm not sure why, as I've never noticed this before?

Does it always happen?

 

[zorinlynx]

I let my battery die on my iPhone 5S and the ringer was also on. When I plugged it on the charger, it took a few minutes for it to come on, but when it had the white screen w/ the black apple, it vibrated, but I'm not sure why, as I've never noticed this before?

Does it always happen?

This is normal. The phone vibrates during the boot process for me too.

 

[Altis]

Happens with mine but only when the phone is on silent, I believe (even though I have vibration disabled).