I've tandem nursed twice, and this happened with both of mine. Think about the taste fo unpasturized, fresh cow's milk (if you've had the pleasure -- heavenly) with the cream shaken in. Add some sugar, plus the yummy goodness fo taking it straight from mama's tap. Yum! Why on earth would I bother eating anything else when I have an unlimited source of that!?
But for me it was just way too much exclusively nursing an almost 2 and a baby. I cut way back and limited breastmilk like a dessert, for after meals, when I was exhausted and becoming anemic. With my first it wasn't such a big deal but caring for 3 little ones plus full time nursing of 2 was just too much for me this last time. For a while we were only nursing at bedtime, my choice not his, though it wasn'ta steadfast "rule" or anything.
There's a great tandem nursing book whose title evades me at this moment; if you can find the time it's worth reading. Title? Anyone?