Border Patrol agents search for immigrants illegally crossing the border from Mexico into the United States near McAllen, Tex., on March 26. (Loren Elliott/AFP/Getty Images)

There was no indication on Wednesday of last week that the White House was about to send National Guard troops to the border with Mexico to crack down on immigrants entering the country illegally. Over the weekend, though, President Trump suddenly started hammering on immigration as a central priority, leading to the administration’s vague, then concrete, announcement about strengthening the southern border.

At the daily press briefing on Wednesday, Homeland Security Secretary Kirstjen Nielsen outlined the administration’s proposal. After she did so, a reporter asked a key question: Why now?

“I think, you know I would say is that the numbers continue to increase. April, traditionally, is a month in which we see more folks crossing the border without a legal right to do so. So partly it’s modeling; partly it’s anticipating,” Nielsen said. “We are seeing more and more advertising, very unfortunately, by the traffickers and the smugglers to our south, specific to how to get around our system and enter our country and stay. We have documented cases of borrowing children, appearing at the border as a family unit in a fraudulent way.”

More and more advertising by traffickers and smugglers may be the case (it’s hard to say), but that point about April isn’t.

Since 2000, there have been a declining number of apprehensions during each federal fiscal year. That year, 1.6 million people were apprehended trying to enter the country illegally. In the 2017 fiscal year, part of which fell under President Barack Obama’s administration, the figure was only about 310,000.

One pattern stands out, though: April is generally when illegal border crossings start to decline, according to data from Nielsen’s own department.

From 2014 to 2017, there were more apprehensions in May than in April, but only in 2014 and 2017 were there more apprehensions in June than in April. The distinct shift in immigration patterns — even in recent years — has been in January and February.

2017 is also anomalous, because it continues the general downward trend in apprehensions but also because it’s clear that apprehensions plunged at the beginning of Trump’s presidency and only slowly started to recover. The Trump administration, you will recall, has taken a lot of credit for that initial dip.

A reporter later asked White House press secretary Sarah Huckabee Sanders a follow-up question on the same subject: “Does this have anything to do with the report that the president saw on Fox News?”

There were reports that Trump spent a good deal of time at Mar-a-Lago over the weekend both watching Fox News and meeting with the hosts of shows on the network.

Sean Hannity and Jeanine Pirro, with whom Trump spoke, CNN’s Kevin Liptak reported, “made known to Trump that his party’s successes in upcoming midterm elections will depend on him being able to tout successes on the border wall.”

We know Trump also watched Fox News because, well, he tweeted about it.

Sanders didn’t answer the question directly.

“I think it has everything to do with protecting the people of this country,” she said. “I don’t think this should come as a surprise; the president’s been talking about securing the border for years since he started on the campaign trail.” The president wanted Congress to pass legislation, but it didn’t, failing “time and time again.”

“Now the president is making sure that, in between the Congress actually doing something, he’s doing what he can to protect the people of this country,” Sanders continued. “And he’s going to continue to do that and look at different measures that he can do that, whether it’s through the National Guard, which is what he’s doing today, or whether it’s through other administrative actions that he has the authority to carry out without having to involve Congress, and say simply — Democrats apparently can’t show up and actually do their jobs.”

Democrats, we will note, are the minority party in both the House and Senate.

We know that Trump met with people who advocate a tougher line on immigration. We know that Fox News spent more of the weekend (March 31 and April 1) talking about immigrants than its competitors.

What we don’t know is why the default assumption should be that this wasn’t the main driver of the urgency of the issue.