How to pack items that may leak liquids for lunch?
For your situation I would recommend packing the burritos individually (or 2 at once) so this does not happen.  You can achieve this by using foil, plastic wrap, or bags.  Alternatively, you may try putting a sheet of wax paper (or foil/plastic wrap - but remove before microwaving) between each layer of burritos.

If this does not work for you, try the following:

Here are two methods that can be used to try to preserve expected consistency:

The easiest, and most sure to actually work:

Store the components that are wet and going to leak liquid in a different container.  Unfortunately, this may involve you having to build your burrito when you want to eat it, yet this is sure to prevent a soggy burrito.

To do this you can use separate containers, bags, or however you normally package a burrito.  One can also purchase plastic food containers with multiple compartments specifically made for this purpose (separation of different food items).  For an example, here is a product listed on Amazon.

The second way:

This isn't a long-term fix nor is it as efficient as a multi-compartment food container.  One can strategically build certain food items so that they are less likely to make bread or tortillas soggy.  You have to place a food item, that will not let the bread absorb the liquid as quickly, next to the bread.  The best example I can give you is that when I make a BLT sandwich that I am not going to eat immediately, I place the lettuce right next to the bread instead of putting the tomato right next to the bread.  The lettuce provides a barrier between the two.  Like I said though, it will not be as efficient as multi-compartment food containers.