Could it be that listerine is the cause of mouth ulcers?

Yes: Listerine contains alcohol and other chemical ingredients that can cause a reaction in the mouth that can lead to mouth ulcers. One of the ingredients, methyl salicylate, has been shown to cause allergic reactions and contact dermatitis. You should consider stopping the listerine to see if it helps.

Possible: First, there are many kinds of "mouth ulcers" and each one has different causes. It's best to consult with a dentist if you have one that does not go away or get them frequently. Listerine can be irritating to the soft tissues of the mouth and may be a contributing factor. Short of that, you can run your own test by stopping it, then resuming it. See if the ulcers coincide with its use.