What are the current (technical) criteria for filtering posts into the review queues?
The FAQ What are the review queues, and how do they work? covers some general discussion on the queues (but no specifics). Also, New Feature: Community Review Tasks - Now in Beta mentions some vague selection criteria ("algorithmically determined...") and targets only specific queues. However, while some queues seem fairly obvious in their implementation, there are still questions as to why certain posts end up in a review queue.
This post originates from a Bug in how new user status is calculated where an existing, well-established user posted an answer to old question yet ended up in a late review queue (designated to catch "new user's answer to an old question"). Admittedly this user is TeX - LaTeX's top investor and frequently dips into the low-rep end of the pool as a result: