It is a good question. What you want is not a crypt but a catacomb. It looks like proper catacombs for mass burials always turn on issues of use of space and use of land. Wikipedia is excellent for the Roman and Parisian and the reasons for the two were very different. I also found reading here: http://mentalfloss.com/article/64564/7-worlds-most-fascinating-and-beautiful-catacombs
I found it stated that the Roman catacombs were built because burial of corpses was illegal in the city. I cannot find out why that was but it is not a bad idea if land is at a premium. I think the main reason that there were galleries upon galleries of burial chambers and passages is that families had an obligation to continue honoring the dead, which they did with fancy art and visits. If you have an obligation like that you want to be sure your neighbors know you are honoring it, and you want your tributes to be out where they can be seen. And: it was the done thing at the time for rich and poor alike.
The Parisian ones were very different. I read that people were buried in the city, for centuries, and it got nasty with heaps of bodies falling thru walls etc. The Parisians had a huge labyrinth of mines handy and so the bones of millions (!) of bodies were exhumed and moved to these old mines. And arranged in artistic patterns all mixed together. They saved the freakiest bones they found for the special freak bone room. Sort of the opposite of the Roman catacombs as regards respecting your ancestors.
The Vienna catacombs are similar to the Parisian: an expedient and space-frugal way to store lots of bodies. I read that the Vienna ones got started after 11,000 people died during a plague and they had to go somewhere.
All these catacombs came about as a consequence of high population density and need for economical use of land in storage / disposal of corpses.
Finally, the under-construction Jerusalem catacombs. This is a different reason again and I think one that would be good for your RPG. Jews want to be buried in Jerusalem. Religious preferences as regards site were also apparently important to the early Christians buried in the Roman catacombs: they wanted to be near the graves of martyrs.
I could imagine in your campaign something important happened at a given site such that for hundreds of years people insisted on being buried there, and to accommodate the demand efficiently, catacombs under (or over) the site were constructed. Times changed and the old religion and old catacombs were forgotten...
/Interesting, but not needed in the forest, and not geotechnical. – RonJohn 8 hours ago /
RonJohn is right on both counts. So: a clarification. One is as regards the forest. If the catacombs have been forgotten, a forest may have grown up. The people currently there are not the same people who built the catacombs. Alternatively, the sacred event that caused this site to be the place for burials just happened to take place in the forest.
Re geotechnical: if you want a grave in proximity to the grave of a martyr, it is easier to accomplish if you have 3 dimensions to work with (multi-tiered catacomb) as opposed to only 2 (graveyard). This was the case for the abovementioned Christians.