[sokoloff]

It's overwhelmingly likely that the person did something that was fireable, even in the big bad, limited employee rights USA.

At the same time, professional discretion is the norm and so you might not know from the written/spoken language if someone was let go because of a specific acute incident or a pattern of under-performance. (Except that people usually know who the chronic under-performers are, so when one is let go, you tend to assume it was performance-related and if someone thought to be a high-performer is let go, you tend to assume a non-performance cause.)