We study the nonlinear effect of a background of microturbulence (in the lower hybrid/ion cyclotron frequency range) on the evolution of a tearing mode instability in the magnetospheric tail region. We find that the linearly stable collisionless tearing modes can be driven unstable by the additional dissipation due to the turbulent fluctuations. The growth times are of the order of 8 to 16 minutes. This mechanism could be important for understanding substorm activity and rapid magnetic reconnection in the tail region.