How is saying "No problem" rude and condescending?

I work in a senior living community taking care of the elderly.
Today I was a helping this old man up his chair and when he said "thank you" I said "You're welcome", then he points at me while looking at an old lady and says "see, she says 'you're welcome'!" Then the lady goes on to say that he hates it when people say "no problem" instead, because it is rude and condescending.
I thought about it for a while but didn't seemed to understand why. All I came up with was maybe the other person is saying it's not a problem like if it were? I don't know...
Can anyone explain?
How is saying "No problem" rude and condescending?
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