A sudden change in actions or reactions

Burst out

To suddenly begin a physical action.

- Every one in the room burst out into conversation.

- The audience burst out into applause.

 Burst into

To suddenly begin a conversation, singing, .

- Everyone burst out into conversation/applause/song.

- At one point, members of the security forces reportedly burst into a meeting of demonstrators.

- he burst out speech when we entered.

Break into

To start doing something suddenly:

- The child broke into a run /a song.

- The horse broke into a brisk trot.

- If she lay on her right side, she broke into a paroxysm of coughing.

Explode into

Suddenly begin to move or start a new activity:

 - A bird exploded into flight.

-  He suddenly exploded into action.