Learn Spanish (Latin America): La multitud se puso a abuchear.

The phrase ponerse a followed by an infinitive is used to describe a sudden, spontaneous, or emotional start to an action. A crowd booing is exactly that kind of sudden, emotional reaction! While empezó a abuchear is grammatically correct, se puso a gives it that spontaneous feeling.

Since a crowd is a group of people, why isn't the verb plural (se pusieron)?

In Spanish, collective nouns like multitud (crowd), gente (people), and familia (family) are grammatically singular. Even though they refer to many individuals, the noun itself is singular, so it takes the singular verb se puso.

Can I drop the se and just say la multitud puso a abuchear?

No, the se is required here. The verb poner on its own means to put or to place. When you add the reflexive pronoun se to make ponerse a, the meaning completely changes to starting an action or setting oneself to do something. Without it, the sentence would sound like the crowd was physically placing something down.

Is abuchear related to the English word boo?