This verse from the "Animal Apocalypse" of I Enoch has been taken to be corrupt, especially the words "into his shadow." Patrick Tiller says "the phrase is peculiar." In his translation, R. H. Charles places it between brackets.
Nevertheless, I think the phrase may have originated in an overly literal translation of Aramaic בטלל, bitlal. This phrase, literally "in the shadow," comes to mean "because of, on account of," in the Aramaic of Qumran, as in these phrases from the Genesis Apocryphon:
שביק ארזא בטלל תמרתא, "the cedar was spared because of the date-palm" (19:16)
ואחי בטליכי, "I will live because of you" (19:20)
Therefore the original of Ethiopic wa-wadqu kwellomu westa selalotu would be something like ונפלו כלהון בטללה, "they all fell because of him."