I have had lots of logs milled and the lumber air dried. Your problem
is common with air dried lumber (similar with what I have experienced,
also), i.e., the wood still has some elevated moisture content. Kiln
drying would help reduce the amount of bowing/warping you are
experiencing. Try cutting the boards at a much slower feed rate.
This will/should help, some.
The wood may also have stress within, so your cutting relieves the
stress and bow one way or the other. If you flip the board around and
cut from the other end, does it bow the same way? If so, then the
main problem is the moisture in the wood, not so much the stress
The cut line is heating up, as you cut, drying out the wood along that
cut line. As it dries, during the cut, it will shrink along the cut
line, pulling itself toward the cut line.... similarly to a wet board
left in the sun (drying mostly/faster on that exposed side) and it bow
or warp on/toward the sun side.
Run the boards throught the saw at a much slower feed rate, make sure
your blade is sharp and make sure the fence is parallel to the blade.