FYI,
To my complete amazement and following me testing many aspects of this issue of mine, i found out for the first time ever that the physical metallic shield of the USB plug , not the minus power line in it, is not grounded. however, checking connectivity between the USB ports shields on the PC itself and outlet power - they seem to be grounded. So what i did was checking the same on laser side and - same. Smnoothie USB plug’s shield is floating. To add to the confusion, I then grounded the shield of Smoothie USB plug (to Laser chassis which is grounded) to earth and now M106 do not reset my board.
The more i learn as i go, the more puzzled I am.