How do some drugs that are muscle relaxants cause convulsions as side effects? doesn't that contradict itself?

That's true:: The main function of that drug is to Relax the muscles, for which its generally used; BUT, as any drug, it might have a side effect (in some individuals) of causing convulsions. In majority patients it does it job, but in some of us, due to how it is metabolized, it can cause side effects. Hope that clarifies it for you. Good Luck.