Whenever we send a spacecraft or rover to another planet NASA is very careful to make sure that it isn’t carrying biological contamination from Earth. It’s important to keep the destination planet pristine and untouched by humans for that planet’s own good and also because it could taint any future studies that we do there.
It looks like it was due to some miscommunication and hopefully NASA will use this situation as a learning experience so that it doesn’t happen in the future. A set of drill bits are the culprits. Project developers made an internal decision not to send the equipment through a final ultra-cleanliness step due to Curiosity’s target landing spot, Gale Crater, which they believe is free of potentially life-harboring ice. That information did not reach NASA’s chief protector of the planets, Catharine Conley, until it was too late. The bits were to be put in a sterile box and not opened until it was on Mars, but it was opened while prepping the rover.