In 304 it is probably choride stress cracking, but the S will make it more likely (lower stress, lower temp, and lower choride levels required).
Even when the nominal loading is compressive there will be tensile stresses some where. If you load a part heavily in compression and then release the load the residual stesses will be dominantly tensile.
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Corrosion, every where, all the time.
Manage it or it will manage you.