I may see some caused by trauma from toothbrushing slipping and catching the tissue, from citrus foods, aspirin may cause it too if it is kept inside the cheek. I believe it may also be caused
in females due to hormonal changes.
Some toothpastes may cause those lesions to appear as well, so as experiment you may want to see if things may improve with switching toothpaste to something else (see toothpastes without sodium lauryl sulfat, ProNamel by Sensodyne come to mind as one).