Girlie, Rifampin puts the liver into overdrive as it is such a difficult compound for the body to break down and metabolise. Because of that all kinds of liver enzymes are elevated, and this is what causes some drugs that are combined with rifampin to have reduced blood levels as they too are metabolized faster. Rifampin induced elevated liver enzymes may do that for tinidazole too, but I don't know for sure.
Some people had elevated liver enzymes when combining the two. The combo just causes too much oxidation and can be permanently damaging. It is not a good idea to combine the two, however there are doctors that will combine them. I think it's good to consider the risk though.