That's right, that is the infamous "Leon Schlesinger Lockout" which occurred in May 1941 (also known as the "Little Six-Day War" in the words of Chuck Jones. This event happened when Leon Schlesinger attempted to lock out the Guild animators from his studio from having his studio unionised. Known as the Screen Cartoonists Guild, it was an organisation in which, under the leadership of Herb Sorrel, was attempting to unionise all animation studios in the U.S. Since the Fleischer studio met with victory in 1940, and studios like MGM, Lantz and Columbia had signed contracts--Warners and Disney were the only studios left who hadn't yet signed the pact.
|From L ro R: Ben Washam, Roy Laupenberger, ?, Paul Marron, Martha Sigall.|
Only lasting six days, Schlesginer quickly relented and agreed to sign contracts, and at that point Leon Schlesinger then reportedly remarked, "What about Disney?", who was the only studio left to not go unionised. Thus, this would lead to the infamous Disney animator's strike, which is another story.
If you want more information, you can read about the "lockout" as well as the unionisation of other animation studios through Tom Sito's excellent book: Drawing the Line: The Untold Story of Animation Unions from Bosko to Bart Simpson, as well as information from Martha Sigall's autobiography, which explain a little more about the event, though it did not impact the studio much at all.
Sorry if this seems a rather abrupt post with information already taken from, but the story about unionisation at the other animation studios are a fascinating part that scarred animation history, consider this that they did change the industry forever: especially the unionisation of Disney. Though it definitely had its advantages as well as disadvantages, but that will be for another time..