Learn Spanish (Latin America): ¡El perro ladró y el gato se echó a correr! - The dog barked and the cat took off running!

The structure echarse a followed by an infinitive verb (like correr) is used to emphasize that an action started very suddenly or unexpectedly. While empezó a correr just means the cat started to run, se echó a correr gives the vivid image that the cat bolted or took off running in surprise.

I thought the verb echar meant to throw. Why does it mean took off here?

The verb echar is very flexible and changes meaning depending on the words around it. When you make it reflexive by adding the pronoun se, pair it with the preposition a, and follow it with an infinitive action verb, it creates a completely new phrase meaning to burst into that action.

Can I use echarse a with any verb to mean starting something suddenly?

No, it is mostly restricted to verbs of physical movement (like correr for running or volar for flying) and spontaneous emotional expressions (like llorar for crying or reír for laughing). You would not use it for a deliberate action, like suddenly starting to read or study.

Why is ladró in the preterite tense here rather than the imperfect?