spring on someone - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Present or make known unexpectedly, as in They sprung the news of their engagement on the family last night. This idiom uses spring in the sense of “make a sudden move.” Mark Twain used it in Tom Sawyer (1876): “Old Mr. Jones is going to try to spring something on the people here tonight.”