Does increasing spending to more than 90% of one's credit limit on a credit card ruin one's credit score? If so why?
It does affect your credit score in the short term. As one of the factors that determines your credit score is the ratio between credit used and credit available .
The lower the percentage of credit used as compared to the credit available the better it is for your credit score . Over utilization of your credit limit shows that you are over leveraged and in case of exigencies the probability of default / delay in payment goes up . Thus the risk profile increases in such case .
However if it's a one off case the rating agency will give low weight-age to such events as time passes.