It's a simple but effective deposition tactic, one that should be used throughout a deposition. First, have the witness develop a list of all possibilities until the witness's memory is exhausted; then return to explore each possibility.
Q. Tell me why you fired X?
A. He did poor quality work, for one.
Q. What were the other reasons?
A. He didn't take instructions well.
Q. Besides doing poor quality work and failing to take instructions well, were there any other reasons you fired X?
A. He didn't turn in assignments on time and he didn't get to work on time.
Q. Other than doing poor quality work, failing to take instructions well, not turning in assignments on time, and not getting to work on time, were there other reasons that you fired X?
Q. What do you mean when you say that X's work quality was "poor"?
. . .
This sort of questioning pattern will work in many of the areas you cover in a deposition. One advantage is that it will keep you organized and on track. A second advantage is that you will box the witness in with a "no" answer (like the second-to-last question in the example) that will serve as an impeaching response, should you need it later.
For other deposition tips like these, see my book Deposition Checklists and Strategies (James Publishing). You'll also find other tips, and more information about the book, in the following posts at Illinois Trial Practice--