If your receiver is switched on but your transmitter is not, it will pick up any faint signal it recieves, like when you switch on the lights, or your coffee machine. Your servo's will react by twitching. When you turn on your TX it will take some time (milliseconds probably) to stabilize, so the first signal that reaches the servo's may be "wrong", this also causes a short twitch. This is all very normal. If it takes more than, say, a second or so, something is wrong and you should let someone take a look at it.

It is good practice to always turn on your transmitter first, and than the reciever. And to turn off the reciever before the tranmitter, it probably says so in your manual. Jumping servo's can damage your plane if they move further than you had planned and they can even damage themselves.
Love,
Gary