The Harlem Shake videos last between 30 and 32 seconds and feature an excerpt from the song “Harlem Shake” by electronic musician Baauer. Usually, a video begins with one person (often helmeted or wearing a mask) dancing to the song alone for 15 seconds, surrounded by other people not paying attention or unaware of the dancing individual. When the bass drops, the video cuts to the entire crowd doing a crazy convulsive dance for the next 15 seconds. Additionally, in the second half of the video, people often wear crazy outfits or costumes while wielding strange props.
So that is why Lambie is wearing a mask at the beginning and the other animals are ignoring her. And why they all start dancing crazy later on in the song.