If you don't wean off of wellbutrin (bupropion) slowly can it trigger mania?

No. Mania, when triggered by an antidepressant, happens by being on it and being at risk for having bipolar disorder. It does not happen from stopping the antidepressant. Some antidepressants can cause withdrawal-related side effects if they are stopped suddenly. However, I'm not aware of any such side effects from wellbutrin (bupropion).
Generally not. Wellbutrin (bupropion) is one of the antidepressant medications that seems to have few withdrawal effects. It also is less likely compared to other antidepressants to trigger bipolar symptoms, especially when the person is not yet on a mood stabilizer. That said, the possibility then of mania as a result of rapid withdrawal of wellbutrin (bupropion) seems unlikely. As with all withdrawal close attention is needed.
No. The danger of mania being triggered by an anti-depressant is not related to weaning but to its use in the first place when there is an unsuspected bipolar condition "lurking" behind what looks like a "unipolar" depression--then an antidepressant, especially used with no mood stabilizer in place, can provoke the mania. Some antidepressants can have rebound jitteriness if stopped abruptly.