I think it's both psychological and neurological if that's the right word. From my experience it started off very light and the more I tried to refine my speech the worse its gotten. So starting off it was a normal part of learning to speak, then it became almost a hobby to try and get rid of it, then as it got worse it became a worried which caused me to do it more and after years of that I think it's become almost like muscle memory for the brain