Confused by a sentence from a children's picture book!
Sergent Murphy was busy putting parking tickets on cars WHEN, SUDDENLY, WHO SHOULD COME RUNNING OUT OF THE MARKET BUT Bananas Gorilla. He had stolen a bunch of bananas, and was trying to escape.

The above words are from a picture book for children. The words written in capital letters are the part that I can not figure its meaning out. All those words are 'easy', but it is hard to guess when putting together.

Question

Confused by a sentence from a children's picture book!
Sergent Murphy was busy putting parking tickets on cars WHEN, SUDDENLY, WHO SHOULD COME RUNNING OUT OF THE MARKET BUT Bananas Gorilla. He had stolen a bunch of bananas, and was trying to escape.

The above words are from a picture book for children. The words written in capital letters are the part that I can not figure its meaning out. All those words are 'easy', but it is hard to guess when putting together.

Answer

'When' means that Bananas came out at the time when Murphy was ticketing the cars.
'Suddenly' means that Bananas came out quickly.
'Who should come out' is a rhetorical question. Because it is written as a story, the writer is asking the listeners to guess who came out.
Does that help?

Answer

A simple way of saying this is  "Sergent Murphy was busy putting parking tickets on cars WHEN SUDDENLY Bananas Gorilla CAME RUNNING OUT OF THE MARKET. He had stolen a bunch of bananas, and was trying to escape."

"who should X but Y" is a dramatic way of saying X happened before revealing that Y is the subject of the action. It's not commonly used. Another form of this may be:
"Who should come running out of the market? None other than Bananas Gorilla!"