Why put lemon juice in a bread recipe?

To adjust the pH (acid/base balance). You could also use vinegar.

If it's a 'quick bread' (i.e. not made with yeast but baking powder), baking powder is a combination of an acid and a base so when it gets wet it makes CO2 bubbles. If you have any acidic or basic ingredient you need to add a little of the opposite to make the baking powder work properly. So for instance if you're making buttermilk pancakes you add a little baking soda because buttermilk is acidic. In your case, perhaps one of the ingredient is basic so you need to add some acid to compensate.