Is it possible to lock an active layer?
Yes, but that only prevents selecting entities on that layer. It does not prohibit new entities.
When I change tools, Turbocad Pro v 16.2 goes to another layer all by itself.
This makes sense for Text and Dimensions but further it is very confusing.
If you change to a tool in a different tool group the active layer switches to the last-set user default layer for that tool group. For example, if you set a dim' tool to draw on "DIM" layer, the next time you use a dim' tool, TC automatically makes the "DIM" layer the active layer.