It is well known that every QBF or first-order formula can be rewritten to a prenex normal form
$$Q_1x_1 \ldots Q_nx_n. \phi,$$
where $Q_i \in \{\forall, \exists\}$ and $\phi$ is a quantifier-free formula called the matrix.
For me, this name seems pretty unusual for a logical formula. So my question is why do we call $\phi$ matrix?
Merriam-Webster definition of matrix: Something within or from which something else originates, develops, or takes form.
Sounds like a reasonable description of $\phi$ in this context.
In fact, it sounds like a better use of the word "matrix" than for "a rectangular array of numbers".