If it is a glass tank with sheer walls I don't see why they should be able to get out. However, if they have a foothold and they are actively climbing up past 6 inches then I agree you have a problem. The cover with the aluminum foil sounds good. And for why they are doing it...well, I guess that they are just weird. :)

I used to purchase crickets from Petco to feed a pet scorpion. Since Petco was a 30 mile drive for me and the cricket inventory was irratic, I used to stock up when I had the opportunity. A five gallon pail covered with cheesecloth with a food supply of oatmeal and fruit scraps was the perfect home. The walls of the bucket were too high for escapes. The crickets were happy in their home and I had populations of baby crickets as well. I think that your terrarium has walls that are too short and permit the crickets to jump out.

Cheers!

The crickets are climbing the silicon in the corners of the tank. As long as you have a lid there shouldn't be any problems ... why are they doing this?... FREEDOM...

Crickets have no brains, that's why they're doing it! =) I hope pin-head crickets aren't the only thing you're feeding your PDFs though. They don't really like bugs with hard shells. Fruit flies are an excellent PDF food, and easy to culture yourself.

As to why, probably overpopulation. If they get free now [ as chris mentioned : ) ] they can spread out before the mature and territorial issues arise.