Because Ephraim has multiplied altars for sinning, They became for him altars for sinning.
"Altars have been unto him for sinning ..." The purpose of an altar was that of procurement of the forgiveness of sins; but in the case of Ephraim, his altars were only occasions for committing more sins.  This derived not merely from the fact of their multiplicity, which in itself was sinful, but also from the fact of the vulgar and licentious "worship" associated with the altars of the fertility cult all over Israel.  Sacred prostitution was their dominant feature.  Having multiplied altars and having degraded them with the evil rites of paganism, the very purpose of the altars, in any holy sense, was lost to the nation of northern Israel.