Here's why:
* 一口吃掉牛尾巴 (yī kǒu chī diào niú wěi bá) literally translates to "eat up a cow's tail in one gulp".
* 告 (gào) looks like a simplified picture of a cow's tail being swallowed. The top part resembles a mouth, and the bottom part looks like a tail disappearing into it.
This riddle plays on the visual similarity between the character and the described action.