Why does my phone vibrate when it turns on after it was off?

Most Androids I have used will vibrate when powered on. This serves two functions:

1. Tests the vibration motor
2. Notifies the user that the phone turned on, in case it was an accident you wanted it off to save power.

First, remove any recently installed apps. If that doesn’t help, I recommend trying a wipe of the cache partition. This will clear out a bunch of temporary data and, although it will cause a slower boot-up the first time after being performed, will usually result in a much more responsive phone later.

Here are the steps:

1. Turn off the device.
2. Press and hold the Volume Up key and the Home key, then press and hold the Power key.
3. When the logo screen displays, release only the Power key.
4. When the Android logo displays, release the Volume Up and the Home key ('Installing system update' will show for about 30 - 60 seconds before showing the Android system recovery menu options).
5. Press the Volume down key several times key to highlight 'wipe cache partition.'
6. Press the Power key to select.
7. Press the Volume down key several times key to highlight 'yes.'
8. Press the Power key to select.
9. When the wipe cache partition is complete, 'Reboot system now' is highlighted.
10. Press the Power key to restart the device.