My former employer has been contacting some of their current employees into their office and questioning them on my reasons for leaving. Then, allegedly, they are telling these employees why they had wanted to fire me but didn't. In addition, they are making up stories about me. Isn't it illegal for an employer to discuss employee termination/resignation conditions?

Nope, its only illegal if they lie and if those lies damage you in some way.

Agree, if they are telling the truth, there is no defense.
You could consider sending them a letter telling them from stating falsehoods against you. Was there an issue, or something you told people prior to leaving ?

No issue other than it appeared that I threatened the office manager. There had been speculation for a long time that I may want to go into business for myself. Recently the owner started to treat me in a very skeptical way. I had an opportunity to go out on my own, and it had been really stressful, so I decided to make the change, and now the owner along with the branch office manager are stirring up false statements about me and telling some of their current employees along with interrogating them as to any other "real reasons" I chose to leave.
But no "damage" has been done. I just didn't think that it was legal to disclose to other employees a former employees reasons for resigning.

There are no laws that I am aware of that prevent an employer from letting other employees know why an employee left. Most employers don't discuss it for fear of law suits.

To sue you would have to prove not only that they lied but that you were damaged by those lies.