What else can it be?

Did u clean the battery terminals and don't forget to check at the point where cable meets the clamp. Follow to silinoid make sure cables r tight and clean follow to the starter and check their dont for get to wiggle the starter to make sure its secure its bolted properly. A few cases were I had someone else do some work on my vehicle and wasnt done properly, loose starter can cause the starter to bind against the flyweel and cause it to drag. If that doesnt work have starter tested