p-columns, then there is a ambiguity: Does
\\ should end a line inside a cell or should it end a tabular row? The use of
\tabularnewline make that clear, the first ends the line in a cell, the latter the row of the table.
\arraybackslash of package
array that can be used to reset the meaning of
\\ to be
\tabularnewline, from the documentation:
\\ for use in array and tabular