If, throughout a GMC, the ambipolar diffusion time
tAD were to instantaneously
drop to a value less than the local free-fall time, the bomb would
certainly ignite.
Since tAD depends directly
on the strength of the magnetic field, one might imagine a
scenario by which the B-field inside the GMC decreases during
a galaxy-galaxy interaction ... but you would be hard-pressed to
find anyone who would buy such a scenario!