Players selected in the first round have high expectations from fans and the team, high risk, and usually large contracts. At the OLB position in a 34 defense (especially in the Patriots scheme), the player is asked to play standing up, read defenses, and drop into coverage a lot on first and second down. I don't think there are many college programs that ask anyone to do the same thing. Thus, there is virtually no college player who has done similar things in college as to what they will be asked to do in the pros at 34 OLB. This contrasts to most other positions in the game: route running, zone blocking, man-to-man coverage, stunts, etc. are all techniques or schemes that are run in college and translate considerably to the NFL.
Ultimately, it is much harder to judge whether or not a certain player will have the necessary physical tools or mental capacity to perform the duties of a position that they have never played before, versus a player who has extensive experience (and thus game-tape) at a particular technique/position/scheme. Because of this, extrapolating a players success at the 34 OLB position is difficult - and there are not many good first year starters at the position. This increases the risk while not reducing the contract size or expectation level.
