Hi MingYou, 

The word "burst" is tricky because its form in the present and past is the same. We don't tend to change the form of the past-tense verb to agree with person and plurality in English (except for the verb "to be"), so the examples that you've given are in the past tense.

Let's look at an easy example: the word "eat".

(i) She eats an apple.

(ii) They eat apples.

The form of the verb changes depending on the subject when we're talking in present tense.

But look at the past tense forms:

(i) She ate an apple.

(ii) They ate apples.

They're the same on the surface.

With the word "burst", you would only see "bursts" used if the sentence is in the present tense (and only with the third-person singular). 

Have a look at these links: 

http://www.verbix.com/webverbix/English/burst.html

http://www.usingenglish.com/reference/irregular-verbs/burst.html