Here's why:
* Past Tense: The past tense of "burst" is "burst," not "burstd."
* Past Participle: The past participle of "burst" is also "burst," not "bursted."
Here's an example:
* Present Tense: The balloon bursts when you blow it up too much.
* Past Tense: The balloon burst in my hand.
* Past Participle: The balloon has burst and cannot be inflated again.