That usually happens when the original trope is too specific for the name. If it's a legit trope (and not just too specific because someone built it primarily around a single example), then the original definition gets split off.
"Preserving the previous definition" typically entails a very tedius example/wick cleanup or the previous definition is The Same, but More Specific of the trope the article is misused for. Either way there is frequently little interest in preserving old definition in these cases.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
No, the other one.
Widespread misuse can be a sign that a trope wasn't properly defined to begin with, and the actual pattern is what the misuse is about. Not necessarily, but it can be.
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Chair Man
Read the Trope Decay page and you'll see that definition drifts like those aren't always bad.
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Lost in Space
Less drinking and troping, please.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
I was moved to recall what happened to Exact Words. It was launched as a comeuppance trope, in which a character establishes rules and policies in their favor, which are later twisted against them with "exact words" in order to teach that character a lesson. But there was so much misuse based on the name that everyone wanted to just change the definition to match what people were using it as. There was no desire to preserve the original definition at all. Something similar happened to Battle Butler (a servant character who obeys a villain out of a sheer sense of duty).
This has always struck me as not an ideal route to take; I mean, those tropes were launched with those definitions for a reason, right? How often has this happened? Are there any other examples I don't know of?