Auto insurance rates, which tumbled after Massachusetts opened the state to more competition four years ago, are marching steadily upward again. Many of the state’s largest auto insurers have raised rates in each of the past two years, citing the need to keep pace with the rising cost of claims for car accidents, theft, and injuries. In addition, some companies say they couldn’t make enough money after dropping rates in 2008 to compete with rival insurers or gain market share.