To summarize/clarify:
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Wind caused tow planes to take off, not gliders - the gliders had lots of warm bodies holding them down.
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The tow planes had pilots in them: note the idling propellers.
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As the tow planes were facing into the wind, the lift generated by the airspeed of the wind over the wings was sufficient to lift them off the ground, even with zero initial groundspeed, after which the pilots revved the engines sufficiently to maintain safe flight/control.
Years back in the (Canadian) Air Cadets, some of us were pushing a glider on the ground in moderate winds (facing into the wind); when pushing it onto the runway over a slight rise similar in height to a speed bump, the extra little bounce combined with the lift from the wind made it start to rise off the ground, perhaps a foot off before being hauled back down by the weight of a number of very scared cadets.
