A really minor annoyance, but strange nonetheless. Worth to investigate.
To reproduce :

• create a north/south railway (no need to be long or powered)
• put a minecart on it
• climb in the minecart
• make it move by pressing forward
• after roughly one second of movement, the player's head will turn spontaneously 45° east (right when running north, left when running south)
• do not happen on east/west railway
• happens only once on brand new minecarts
• happens only when minecart start to move
• not affected by view mode (F5), game mode, difficulty, etc.

I've seen this in every version as far as I remember. Maybe it is intended behaviour...

I will be perfectly ok if this is not corrected ...