Teams only get a set amount of money to sign their draftees. The first few rounds have picks that are given a slot value. The first overall pick gives the team like 8.5M towards their slot. The Sox had 6.6M for the 4th overall pick. If a player wants over slot, you have less money to give the other picks. If you sign a guy under slot, you have more money to spend on other picks. Some guys fall because they have a high $ demand. Some guys go earlier because they’ll take under slot.
For example, Jud Fabian in the 2nd round probably wants over slot because he was projected to go in the first round. Players are more likely to go back to school after being picked so they do have some leverage.