With Garrett, the reasons are a bit more complicated. Garrett as a congressman vocally criticized and supported shutting down the Export-Import Bank when the issue tore the Republican Party apart. Trump had also criticized it, calling it the “Bank of Boeing,” and his selection of Garrett had long been a sore spot for senators such as Scott, who come from states where companies such as Boeing are vital to the economy.
What's notable here is that these senators and congressmen were basically forced to go public with their opposition before the White House would back down. Rather than handling these matters quietly and behind closed doors, the members rebuked their own party's nominees — and by extension, the president who nominated them — through their comments and votes. Whether that's because the White House refused to back down (Kennedy indicated a few weeks ago that it wasn't listening to his concerns) or for some other reason, it suggests a growing willingness to buck the president from some unlikely sources.
It's also worth noting just how rare Garrett's defeat was. It's actually the first time in more than three decades that a president's party controlled the Senate and defeated one off his nominees in committee. (This doesn't account for nominations that were pulled before a vote could be held.)
Not that this is necessarily a massive embarrassment for the White House; nominations such as these going down are usually one-day stories, and Trump's judicial nominees have glided along at a record pace, with 12 appeals court judges confirmed this year. But the proximity of these examples and the very public complaints about Trump's nominees from GOP senators suggest they're not terribly concerned about alienating 1600 Pennsylvania Avenue on some lower-profile issues — especially when those issues are important to their constituents and supporters.
This week will very probably wind up being a high-water mark for Republican opposition to Trump's nominees. We'll see how much the White House fights back and whether this is a sign of things to come.