Usually we see an upset (buckle or ripple) on the bottom of the tubes if a front end collision is enough to lead to this, but that isn't the case 100% of the time.

Other possible causes are misalignment when built leaving residual stress in the tube, ie. you brazed the D/T first then pulled the head in to meet a slightly short top tube. There's also the perverse irony that a thin larger diameter tube may be more prone to cracking than bending when over stressed.

Those are all possible contributing factors, but my money is on heating/cooling issues compounded by the rough service. Possibly either alone might have been OK, but the combo took it's tool.