I wouldnt say a substantial percentage of weed smokers suddenly get bipolar, come on now. But I do think a tiny tiny percentage of people can be affected by chronic use.
One of my best friends was part of our "smoking circle" in college, we smoked weed daily like everybody else back then, and one night he just lost it, got a panic attack in the middle of everyone's high, had to be hospitalized and it stayed with him (the panic disorder) forever.
One can say he had the underlying issue all along and the weed brought it out, I guess you could speculate it created it, which I doubt. I think weed enhances your underlying personality and sometimes can take a person who has underlying/unresolved emotional issues and really fuck with their head.