According to guitarist Lee McKinney, it's because the band felt as though the new record was incomplete and have re-entered the studio to add on three more songs.
McKinney does not give a tentative release date, though given they've just hit the studio for more recording, I wouldn't expect a final product until 2019. No word on if "Silence The Echo" was just a standalone thing or will be included on the forthcoming album.
Born of Osiris album update – While I thought I finished the album in December, we have continued to go back and revisit it. We added three more songs to the original ten. Sorry for the wait everyone. In the end, you will all be getting a lengthier, more cohesive final product.
— Lee Patrick McKinney (@LeeOsiris) October 2, 2018