Perception of the Rabbit
Alice thought the rabbit was strange because of several reasons.

Unusual Behavior
The rabbit was behaving in a very peculiar manner by talking to itself and constantly checking its pocket watch. This behavior was quite unusual and caught Alice's attention immediately.

Walking and Talking Rabbit
The fact that the rabbit was not only walking on two legs like a human but also talking in a coherent manner was very unusual for Alice. In her world, animals did not typically possess these human-like qualities.

Wearing Clothes
Furthermore, the rabbit was dressed in clothes, which was another factor that contributed to Alice's perception of its strangeness. Animals in her world did not wear clothes, so seeing a rabbit dressed in a waistcoat and carrying a pocket watch was definitely out of the ordinary.

Disappearing into a Hole
To top it all off, the rabbit suddenly took out the pocket watch, exclaimed that it was late, and then disappeared into a rabbit hole. This sudden disappearance added to the mystery and peculiarity of the rabbit's behavior.

Conclusion
In conclusion, Alice's perception of the rabbit as strange stemmed from its unusual behavior, human-like qualities, clothing, and sudden disappearance. All these factors combined to make the rabbit a curious and intriguing character in Alice's adventure in Wonderland.