For many, the cause is neurological. Information have a harder time being transmitted along neurons for us than for non-stuttering people. Sadly, stress-induced adrenaline (among other messengers) interfere with information transmission as well which makes the psychological (or you can call it mental) aspect of it an important part. Our overall well-being impacts our stutter. If we feel like shit, we stutter more and feel even shittier. Other way around works as well.