"What's the occasion?"
I usually like to answer with a smile and say, "No reason... just felt like baking a cake...". I think it paints a nice picture in my co-workers' minds of how I must spend my weekends.
Sometime I mix it up and say, "I bake when work is stressing me out." or "I'm about to get my period." I find these excuses more relatable... because they're true... but maybe not as work appropriate?