Donald Trump seems to think Mike Pence is plotting against him to invoke the 25th amendment and force him out. That might actually be true. However, doing so requires a majority of the cabinet members to agree that the President is unable to fulfil his duties. The reaction we see is that potential threats are being neutralized by Trump by firing people in his cabinet who are less likely to support him, or shuffle them around a bit so they are no longer formally a part of his cabinet.
He might be able to fend off the 25th, but the end is coming, and Trump knows it.