Both of the servos I have (2 diff brands) do something similar. They (or at least one) actually give 2 little twitches before a bigger twitch, then go to either 900 or wherever I have them positioned in setup, but sometimes it will be just three little twitches before default. The only thing that seems to make a difference (maybe) is how much torque is on them at power on. They also twitch every time I turn on the serial monitor.