  We propose that the backbending phenomenon can be explained as a result of
the disappearance of collective $gamma$-vibrational mode in the rotating frame.
Using a cranking+random phase approximation approach for the modified Nilsson
potential + monopole pairing forces, we show that this mechanism is responsible
for the backbending in $^{156}$Dy, $^{158}$Er and obtain a good agreement
between theoretical and experimental results.
