The words This is why refer to the incidents mentioned in verses 3 and 4. Good News Translation takes the whole verse as a kind of aside and encloses it in parentheses.

The priests of Dagon are the Philistine priests, who offer sacrifices to the idol Dagon.

All who enter the house of Dagon: Good News Translation makes clear that the reason people enter the temple of Dagon is to worship him.

The threshold: see the comments on verse 4. In keeping with its rendering of verse 4, Revised English Bible says “do not set foot on Dagon’s platform.”

To this day, that is, at the time this account was written. For the same expression elsewhere in 1 and 2 Samuel, see 1 Sam 6.18; 27.6; 30.25; 2 Sam 4.3; 6.8; 18.18.

At the end of this verse, the Septuagint adds the words “because stepping over, they step over.” This is the basis for the addition in New American Bible “they always step over it.” at and Osty-Trinquet also follow the Septuagint: “they leap over it.”

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on the First and Second Books of Samuel, Volume 1. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here .