There seem to be two options:
The feudal lord might get killed (liquidation in the criminal world). However, since the alternative is "he becomes a business man", which implies an active choice, this seems unlikely - and a bit overly aggressive. Although this is the interpretation of choice when we look at the French Revolution.
Another option is liquidation in the economical sense of the word, which indeed means conversion into cash.
Liquid assets are assets that are readily available for spending or investment - you can read it as "cash money" (yes, it is more complicated economically, but "cash" works for now).
A feudal lord would have many possessions that are not liquid, especially real-estate.
In an industrializing society, that real-estate (which as farmland or otherwise provided income) becomes valuable for other reasons (availability of natural resources, or building land).
So if the feudal lord does not convert his assets into whatever can make him thrive in the industrialized world (so, he becomes a business man), his only other options is to sell his properties, that is, to liquidate. With the proceeds of those sales, he can then live as a financially independent person outside the industrial economy.