The overall key driver is the time of application. However, other factors can over-ride this, for example:
1) Candidate has an exploding offer with another firm: in this case, the recruiting team may do what they can to accelerate the process and interview the candidate ASAP
2) Candidate's compelling personal reasons: e.g. if a candidate is from a foreign country and will be leaving soon, recruiting may see this as a reason to "bump them up"
It is true that offices are super busy, and it can be very difficult to find time to interview candidates. I would definitely not interpret the delay to assume that it means you are a "low priority candidate" or anything like that. Just make use of the extra time to keep prepping!!
Best of luck!