Workhorse Allison Jumped from 5 to 6 Speed (on its own)

My Workhorse chases has a toggle button by the driver window for overdrive - has 3 settings for either 4th 5th or 6th gear to be the top gear the transmission can go up to. Sometimes I put it in 4th going down hills and only put it back in 5th. Revs are usually a little higher reminding me to hit switch again so 6th operates.