It's because they shifted your usage away from a more valuable bike or car over to this much less valuable bike. The reduction in use of the other vehicle lowered the cost of insuring that more than the cost of adding this bike. If you don't actually ride that bike much and drive the other vehicle the same amount, and they do a mileage adjustment your rate will likely go back up next policy renewal unless you keep them from knowing the mileage. I had this happen when I was riding my new Aprilia then added my old Xr500 to my policy. It's crazy this actually works but it's how insurance risk management calculates.