The role of stochastization of magnetic field lines is analysed in fast reconnection phenomena occurring in magnetized
fusion plasma during various conditions in the ASDEX Upgrade tokamak. The mapping technique is applied to trace
the field lines of toroidally confined plasma where perturbation parameters are expressed in terms of experimental
perturbation amplitudes determined from the ASDEX Upgrade tokamak. It is found that fast reconnection observed
during amplitude drops of the neoclassical tearing mode instability in the frequently interrupted regime can be related
to stochastization. It is also shown that stochastization can explain the fast loss of confinement during the minor
disruption. This demonstrates that stochastization can be regarded as a possible cause for different MHD events in
the ASDEX Upgrade.