I actually have the documented answer to your question.
NPR: Thomas Massie files articles of impeachment against Defense Secretary Pete Hegseth
Massie first sought to bring the impeachment resolution to the floor before the midterm elections, prompting Speaker Mike Johnson, R-La., to call an early recess to avoid the vote. That caused the resolution to expire, which is why Massie reintroduced the resolution Monday.
So literally the reason why now is because he tried to do it before, and Mike Johnson maneuvered to delay it. Otherwise, this would have already happened.
And note that this impeachment isn’t in response to the midterm results, because Massie had tried even before the midterm to do this. Hegseth and Massie have been feuding for some time. From the NPR article:
Massie has long been a staunch critic of Hegseth. During the GOP primary, Hegseth campaigned on behalf of Ed Gallrein, who went on to defeat Massie.
In response to Hegseth calling him a “disgruntled defeated congressman,” Massie told reporters on Monday: “Well, he’s right about one thing — I’m a defeated congressman. I’m not disgruntled.”
Massie is also reportedly trying to see if Congress actually has any teeth in regards to reining in an Executive that is going rogue militarily, since presidents have considered the war powers resolution to be “more what you’d call guidelines than actual rules” (to paraphrase Captain Barbossa of Pirates of the Caribbean). And this isn’t just Trump, but the majority of presidents going back for decades, including Democratic ones.
Massie said the effort also underscores the need to litigate whether Congress has real war powers or not. The war powers resolution has largely been ignored by presidents of both parties.
“What’s at heart here is the balance of power between the executive branch and Congress,” he said.