This may or may not be the case, but is it possible you accidently switched to using the Blob Brush tool?
According to the Illustrator Default Keyboard Shortcuts, pressing Option while using the Blob Brush tool will switch to the Smooth tool.
This is assuming you haven't adjusted your default shortcuts.
That would actually be a reason I might accept, if it were true..
But nope.. I was using illustrator.. and I work mostly in Ai, so I'm used to the eyedropper shortcut. To have it disappeared like that freaked me out.
And nope, I was and still am using the Brush tool..
I haven't changed anything in terms of the shortcuts or preferences. =(
In 20+ years... Option/Alt has never caused a brush to switch to the eyedropper tool, in any version of Illustrator, using any brush. You can't even set Illustrator to toggle the two in this manner if you wanted to.
The only explanations are..
you are remebering Photoshop (or some other application), not Illustrator.
It was not switching for you in Illustrator via the Option/Alt key.
You've got some third party software which was toggling the i and Shift-B commands for you.
If you got AI to toggle the eyedropper with the brush, I'd be interested to know how... as I'm sure many other users would be as well. If you sort it out, please share.