In this video from (SpaceX's BFS landing) there is a point where the ship tilts from horizontal « belly first » terminal velocity freefall, to vertical « engine first » landing position.
The graph on the right shows that this leads to a vertical acceleration roughly from mach 0.26 to mach 0.31 induced by drag reduction due to tilting manoever itself.
Gimbaled engines are only fired when the ship is vertical, which could mean this whole tilting manoever is meant to be aerodynamically achieved.
Actuated fins and canard fins are designed to increase or decrease drag about center of gravity of the ship, like a skydiver does by moving its limbs.
But as soon as the ship starts tilting from horizontal to vertical, its body and actuated fins will generate lift. (instead of only drag)
The ship will start gliding backwards. Lift will increase to a maximum point when fins aren't stalled anymore.
Question is :
At this point (gliding backwards, almost vertical, mach 0.3, short before suicide burn) the whole ship should become unstable if center of lift is between center of gravity and aft part of the ship (engines) It's a bit like imagining one backward flying VariEze, or the aerobatic figure called "tailslide" : It will flip over, the same way a badminton shuttlecock does when it changes direction.
When lift becomes significant, what prevents the ship from naturally tilting back to horizontal or abruptly yaw or roll to some unpreditable new attitude (shuttlecock like)?
I agree if cg is very far back, inward folded aft fins and canard fins could create the correct pitching moment for the BFS to tilt vertical. But where should the cg be for it not to tilt about yaw axis, (or induced roll due to the massive fixed vertical fin) Some airliner trading a vertical stabilizer on its tail for one above its cockpit wont be very stable about yaw axis either.
Note that when looking at animations provided by spaceX, aft fins seem to have only one degree of freedom : The huge hinges allow changes in dihedral.
Canard fins seem to act the same but may have one more (invisible in animation) degree of freedom: variable incidence, which could add control to pitch and roll, but still is useless for yaw control.
Edit: please focus on yaw axis, consider seconds 1105 to 1107 in animation. Canard fins don't play any role in stabilizing yaw. How can the aircraft not tumble because of yaw instability and induced roll?