When the black point or white point clipping indicator is on, you see a white outline around the squares at the top corners of the histogram. That’s because you can also toggle the clipping warnings by clicking on those squares. The left one is black point clipping (blue indicator on image), the right one is white point clipping (red indicator).
Choosing View > Show Clipping or pressing J always toggles both indicators. The fact that your screen shot shows only the left one highlighted means that at some point, you probably clicked the left square unintentionally, and that’s how it came on.