This paper proposes a model attempting to explain the appearance of fast growing global disruptive instabilities in diverted configurations when the safety factor near the magnetic separatrix approaches two. We show that if edge density and pressure gradients are strong enough, although external kink modes are stable, poloidal harmonics may couple allowing for a global fluid perturbation to become unstable. The instability window is approached sharply as the mode resonance occurs in the edge region, and is characterised by rather large growth rates even with modest gradients.