Hmm... intriguing. Sorry mate. Nothing pops to head at the moment.

My next suspect is either,
A) You've recently installed an add-in that's mucking up your Excel.
B) Excel is mourning the loss of Steve Jobs.

A: You can answer that yourself.
B: Mind the cliche but, have you tried turning Excel off and turning it back on? Sometimes rebooting the whole pc/system helps too.