From Middle Dutch -drecht.
Van Berkel and Samplonius (2006) derive this element from *drifti- (“creek, waterway”) (related to modern Dutch drijven (“to float”), following earlier scholars.
Van Osta (1996) however considers this unlikely for various reasons; for example, the ft to cht change that this view supposes is contradicted by the fact that in earlier attestations and in dialects where this change did not happen until much later, -cht is always found instead of the expected -ft. In his conclusion he follows Pottmeyer (1929) in his view that the element likely is to be reconstructed as a noun derived from *draganą (“to pull, draw, carry”), but unlike Pottmeyer (who interprets it as a portage), Van Osta believes it refers to a waterway with a towpath.
The old explanation, that -drecht is derived from Latin traiectum (which is the case for the toponymic element -trecht in Utrecht for example), has been rejected by many scholars (due to the unexplained change of tr to dr, among other reasons), though the claim is still repeated by some authors. Van Osta treats this at length.
Also see hypotheses in Ancient Belgian language which suggest substrate influence.
-drecht