It is shown that there exists a nonideal collisionless fishbone mode in toroidal plasmas, which may lead to the formation of the magnetic islands and thermal crashes during fishbone activity. The mode arises because of the resonance interaction between the energetic trapped ions and the collisionless m=1 nonideal kink mode. The theory predictions are found to be consistent with experimental observations of fishbone oscillations accompanied by thermal crashes on the Axisymmetric Divertor Experiment Upgrade tokamak [Günter et al., Nucl. Fusion 39, 1535 (1999)].