Insurance carriers regularly change their rates for any given industry. The reason for this is, when you sign a policy, that policy is put into a risk pool. The longer that pool is active, the more claims are made against it, which increases the premiums for all members of the pool.
When a carrier is building a new pool, they put out aggressive rates to draw in policies. If you have renewed the same policy for more than 2-3 years, you are almost guaranteed to be paying more than you should be.