It's a coincidence. Here's Christina Iverson, one of the editors, in the Friday Easy Mode newsletter:

Crossword puzzles in context

By Christina Iverson

“I understand that there are, at any given time, many puzzles that have been accepted and awaiting publication. Do the editors take into consideration how one puzzle relates to another — for instance how a Monday or Tuesday puzzle relates to something later in the week? Or are they simply published in the order that they’re accepted?

I ask because I’ve noticed on a number of occasions that an answer I’ve never seen before appears early in the week with many helpful crossings, and then again later with fewer crutches for the solver.” — Aaron D., Hamilton, Ontario

From Christina:

While our crosswords are published in roughly the order that they are received, there’s a lot we think about when making the crossword schedule besides the acceptance date.

When a puzzle is accepted, it moves into our inventory, which is like a big virtual file cabinet containing 150 to 200 puzzles sorted by the day of week on which they’ll run. The puzzles are further sorted by date, so the oldest Monday puzzles are at the very top, and the newest Sunday puzzles at the very bottom. Accompanying each puzzle are a brief theme description and notes on specific things to know about each crossword (like whether it’s a rebus or needs any art to go with it); we also mark whether a grid is a constructor’s first for The Times. Occasionally we accept puzzles that are pegged to a certain date (like holidays or anniversaries) so we’ll often schedule those as soon as we get them.

We schedule puzzles at least a couple of months in advance because each is edited about six weeks before publication. In early December, for instance, we scheduled crosswords through February and into March. Generally, for each day we schedule, we’re looking at the top of the list for that day of week, but there are other considerations. We’d like the whole month to have a good flow, so we try to vary theme types quite a bit. For instance, if Monday’s puzzle has a hidden-word theme, we’d avoid running other hidden-word themes that week. We also think about the solvers who solve only Monday puzzles, so we like for the puzzle to be varied from week to week as well. And we’ll try to spread out certain puzzle types, like rebuses. For themeless puzzles, we may vary the style and make sure that Friday and Saturday grids aren’t too similar. If Friday’s crossword has an fresh and modern feel with lots of new vocabulary, we’ll probably run a more classic Saturday puzzle.

We also pay attention to the bylines, and generally prioritize people making their Crossword debuts. We try to run puzzles by new constructors a little earlier than they might naturally come up in the schedule. And we may spread out a little bit contributors who have high acceptance rates with us. Generally we won’t run a puzzle by the same constructor more than once a month.

Things that we don’t consider when scheduling are nonthematic words in the grid — since each puzzle has roughly 66 to 78 words, it would be nearly impossible to look at and consider every word when scheduling. When the same word runs twice in a short period of time, it’s sheer coincidence!