When you have many contact records enrolled into a sequence, and they are sitting In-Queue, the system will randomly select who it sends to.
It doesn't, for example, go right down the list and send in alphabetical order, and it is completely random. You'll notice this is the case when you are inside the Enrollment tab of the sequence and when comparing the Active enrollees versus in the In-Queue.
If you are looking to prioritize certain individuals in a sequence, to ensure they get enrolled and move into the sequence first, you may either clone the sequence and enroll those priority contacts into the cloned sequence, or, simply enroll the priority contacts only into the sequence first and allow them to begin the sequence before enrolling the rest of your list.