If you read the rest of the article, you would see that the union is alleging that the UC administration was not willing to engage in negotiations to raise compensation for grad students, who were unable to afford their rent payments. It's no wonder a wildcat strike happened.
Also, I am saddened by your comment about withholding grades being an unnecessary "burden". That's the point of a strike. What do you think a strike is? They are withholding, not erasing grades, and the goal of the strikers is to force the university to pay attention. The true "unnecessary burden" is UC Santa Cruz not paying grad students enough to pay their rents. If the university had done so, then the grad students would not have had to try to get the administration's attention.