I would guess DeCoteau realized he wouldn't a great deal of playing time. I hadn't heard anything about Dulaney and, as you noted, we knew Butler and Anya-Gafu weren't going to be with the team.
Some newcomers that I wasn't aware of:
Jordan Christianson, a punter from Hurst, TX
Jay Daggett, a WR from Naples, FL
William James, a DB from Sweden (interesting!)
Bobby Gutierrez, a PK from Gainesville, TX (he is listed as a freshman but he apparently was at Northland CC)
Pat Brown, a WR from North Pole, Alaska (also interesting!)
T.J. Draeger, WR from Fargo North
Marcus Laverdure, DL from Belcourt
Some of these guys are listed as redshirt freshmen so I don't know if they just showed up on campus between semesters are if they were with the team last fall but not on the roster.
Some notable changes in size (wt) from last year's fall roster:
WR Ivery +13 to 169 (added weight/strength some help)
RB Garman + 7 to 177 (same as above)
TE Nichols +20 to 245 (he is now TE size - I expect him to have a good year)
DL Jay Nelson -13 to 245 (hopefully, a better playing weight for him)
LB Bennett +16 to 234
OL Hellman +42 to 302 (the Red River product must have been busy pumping weight and eating)
OL Meehan +16 to 306
OL Gillson +27 to 342
DL Henson +12 to 298
OL Windfeldt -14 to 296
OL Hoplin +20 to 265 (another RR product gaining some serious size)
OL Deneui +11 to 304
WR Jackson +23 to 233 (he should be tough to bring down)
WR Young +13 to 198
DL Cummings +29 to 283
TE Adler +22 to 237
DL Woodsmill +19 to 234
DL Brenneman +12 to 266