An argument that contains contradictory premises is trivial in the sense that there is a valid argument from the premises to any well-formed formula whatsoever.
An argument is valid precisely when its premises logically entail its conclusion.
In particular, an argument with contradictory premises is invariably valid (regardless of its conclusion).
It is in this sense that an argument with contradictory premises is said to be trivially valid.
Analogously, the trivial solution of a homogeneous system of linear equations is the zero vector, because this solution invariably arises as the problem’s parameters vary.