Usage and Grammar
Q. What is Chicago’s view on “all of the sudden”?
A. CMOS is silent on the issue, but “all of the sudden” is not idiomatic and normally would be edited to “all of a sudden.” You can compare the frequency and longevity of these two expressions in published books at Ngram Viewer. You aren’t the only one to have noticed the new popularity of “all of the sudden,” by the way. You can read one discussion of the phenomenon here.