How to add recoil to 2d shot

Probably easiest to use an animation so you can make it look like any curve you like.

Alternately if you do it in code, use AnimationCurve objects to drive the rotation and offset.

Beyond why it makes a sudden jump backwards, sounds like a bug.

How to report your problem productively in the Unity3D forums:

http://plbm.com/?p=220

If you post a code snippet, ALWAYS USE CODE TAGS:

How to use code tags: Using code tags properly