I have no insider's knowledge - this is just a guess.

Wayne Kirby - transfer from Oregon; must sit out a year
Joe Tukuafu - signed with Utah State before his mission, and they won't release him
Mika Tafua - gets back from his mission in July
Tanner Baker - playing a year before his mission; however, we are stacked at DE
Brady Christensen - 240 pound OL who gets home in July - needs time to get some meat on his bones
James Empey - got home from a mission last month and weighs 260; needs some time to get back in shape and gain some good weight
Tristen Hoge - must sit out a year after transferring from Notre Dame
A.J. Lolohea - returned from a mission this past spring and at 230 is a but undersized for a stacked DE position group
David Lui - returned from a mission this past spring and will be buried on the depth chart at DE
Khyiris Tonga - returned from a mission this past spring; not sure how much DL he played in high school, though I recall one of the coaches saying he played TE in high school and gained a bunch of weight right after his senior season; will take some time to get back into football shape and learn the position
Jeremiah Ieremia - got back from a mission this spring and supposedly gained a chunk of weight while out; some say he may change to DE, which would be further reason to redshirt him
Kody Wilstead - seems like he would likely be the third-string QB; provided there are no injuries hopefully that means he gets a chance to redshirt

Another exciting thing about the program is the quality of players who are redshirting.