Plaster Crumbling at Base of Chimney

I live in an old house, circa 20's, that has a finished basement. To the rear of the boiler is a plastered wall which apparently abuts the chimney sump. For as long as I can remember, the plaster was intact and held a coat of decorative paint with no sign of deterioraion. Over the past year, the plaster has started to crumble. I was away for the summer and returned to find it had gotten much worse. We have had several severe rainstorms here over the summer, but the fireplace is bone-dry. Could anyone venture a guess as to why water might be getting into the sump but not the fireplace? Or perhaps there is another reason for the erosion other than moisture. May I have your thoughts? Thanks. Frank
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