Rarely, my Alt button gets pressed and it remains pressed though the key comes up at the same height like every other key. Then, it is as good as operating the keyboard with my Alt pressed. To fix it, I hit both by Alt keys in hard so the condition is rectified. It is likely one of your other keys is depressed? Maybe space bar?
...and now I find I don't need to press ALT to resize my brushes, I just press [ and ] by themselves.
To resize brush size, I do not have to press the Alt key in addition. I just press the square bracket keys. It is likely that your preferences were differently selected and not the PS default to allow a variation.
To check up, go to Edit > Keyboard Shortcuts...
Under Keyboard Shortcuts, select "Tools" in the "Shortcuts For" pull down selection and scroll down to see what it shows for "Increase/Decrease Brush Size". The screenshot shows mine so you can compare.