Double Tall Grass turns into peony on break

What I expected to happen was...:
The grass would just break and nothing will actually happen.

What actually happened was...:
I breaked the double tall grass on the bottom part of the block, and the top of it changed into a peony for 1-2 seconds and than disappeared.

I got a screen of it here:
http://oi60.tinypic.com/wi3jat.jpg, you can also check it in the attachment(s)