Most common reason is improperly balanced wheels, next would be alignment but that usually causes a pull to the left or right.
Tires and/or wheels out of balance?
Your power steering pump and or steering box may be about to give out on you.
Loose or worn steering parts, suspension worn, tires out of balance.
balance your tires
Worn steering or suspension parts. This can be very dangerous, so have this repaired ASAP.
There are several causes of this but the most common we call a head shake. This is when the tires shake side to side, the cure is to insure the steering is tight and you add one or two hydraulic steering dampers.
Wheels are out of balance
Sounds like wheel balance, but check the steering dampner.
have front suspension checked
Tire out of balance, bent wheel, drive shaft bent, loose or worn steering parts.
Many things can cause steering shake. Wheels out of balance. ( including rear wheels ) Tryes out of round. Worn steering components. Worn suspension components. Even loose wheel nuts/bolts. Get it checked out for safetys sake.
Tire out of balance, tire defect, bent wheel, defective wheel bearing, loose lug nuts, or worn suspension or steering parts.