Quoting from The Handbook of Glider Aerobatics by Peter Mallinson and Mike Woollard.

"Effect of Airbrakes
Contrary to popular belief, use of the airbrakes does cause a significant increase in the loads applied to the glider, requiring a moderation of the Flight Envelope in a similar way to the ailerons..The reasons are twofold:

(a)the airbrakes destroy lift over a fairly large inboard section of the wing causing the spanwise wing lift distribution to move outboard. This substantially increases wing bending moments.

(b)the airbrakes also generate drag loads on the wing, a proportion of which become an additional load in the pitch plane direction.

JAR22.345 specifies that the maximum positive load factor limits should reduce to not less than 3.5G with airbrakes fully deployed.

It is nearly always better to slow a glider by pulling G rather than opening the airbrakes."