It's developmental. Kids all go through phases of preferring mom over dad, or dad over mom. Both phases will come and go throughout life. Get used to it :) My husband and I always laughed and said, "well, looks like I'm the one who's chopped liver right now." Our son is now nearly 15; I teach 3-6 year olds. I assure you it is purely developmental, and actually a good developmental neurologist could probably explain to you WHY these periods happen.
There is a great book called "Your Child's Growing Mind" though I forget the author, but she's a neurologist. It gives a LOT of great information about brain development, how playing with blocks is actually a pre-math activity, etc. Another great series of books are put out by the Gessell Institute of Human Development. They are: "Your One Year Old," "Your Two Year Old," etc. They're slim books, quick easy reads, but great information for parents who are trying to figure out whether something is developmental or not. It's also great to help you anticipate what is coming up for you -- for example, somewhere around 5-6 there are some seemingly annoying 'behaviors' that are actually developmental, like potty-talk (with giggling), relentless focus on what's Fair, or Not Fair. You don't necessarily dismiss some of these things like the inappropriate language -- because it's your job to teach them what is acceptable and what's not. But it helps to know when the potty talk starts, that your kid is normal and not a 5-year old delinquint :)
Have fun. Enjoy a little "time off" while you're chopped liver, because the day is coming when ONLY YOU will be acceptable to her, and then you'll probably be longing for some temporary relief from the Daddy Phase.