Premature downshifting on downhill

[protegefreak]

When going downhill on the highway, even on not too steep downhills, and i am not stepping on the gas, my 2007 CX-7 with 112k miles downshifts prematurely and sometimes feels/sounds like a thud (like being rear-ended). Result is like the vehicle is engine braking or staying in low gear when it should be at a higher gear.

Any ideas why? Thanks in advance.