The claim of "reaching" is a concept created by people with nothing invested in the selections.....most often by sports reporters or bloggers. In reality, there is no such thing.
When PC and JS first arrived in Seattle, they provided some insight into their drafting concept....the same concept used by most, if not all, teams. I tried to find a video of this, but my Google Fu is weak today. Anyway, here is the gist of it...with a little paraphrasing on my part:
1. All players in the draft are ranked from most desirable to least desirable.
2. This ranking is done without consideration of position or "position-of-need" for the team.
3. However, if 2 players are determined to be of equal talent, then position and/or position-of-need may be the deciding factor in determine their ranking.
4. When it is the team's turn to select, the player highest on the board (highest ranking) is selected....regardless of need. The idea is that every player has value....even if the team does not have a need at that player's position. If the player is not better than the players you already have at that position, he still often has trade value later...especially if they were a high to mid-round pick.
5. IF the top few players that are highest on the board are NOT a position-of-need, a team might try to trade down.
6. IF a player is near the top of the board and fills a position-of-need, but will likely not be available at their selection position, the team might trade up.
7. Trading up or down in the draft is NOT always a possibility and should never be counted on.
8. IF the top player on the board fills a position-of-need....no matter where they are projected on being drafted, a team would be foolish not to select them.
9. Teams rarely pay any attention to sports reporters/bloggers and where THEY think a specific player SHOULD be drafted.
Bottom line, it is all about the board. Teams spend months before the draft evaluating every player and ranking them so that they do not need to figure out who would be the better selection while they are on the clock. When it is their time to select and do not have a trade in place, the top player on the board is often selected.
EDIT: To add to the above:
If positions-of-needs are not filled with FA signings or draft picks, they can often be filled as other teams release player because of cap concerns. Some very good players often become available (and sometimes cheaply) after the draft, as teams try to sign all of their players.