Cracked Transmission Case and Fluid Drained Overnight

I am posting this here for Lori who submitted it through the comment form.

I have a 2003 toyota matrix with a manual transmission. It was not going into/coming out of reverse when I wanted it to. Took it to jiffy lube, was told the transmission fluid was full, but very old. They said it should have been changed a long time ago, but I always have my car checked at jiffy lube and apparently they were just topping it off because he said the transmission fluid was FULL TO THE TOP. That night, drove home fine. The next day, had terrible shifting problems and a burning smell. Drove it to the mechanic the day after that. Much worse. Was told I have a cracked transmission case and all the fluid drained out! I do not understand that string of events at all, and this is costing me $2500, as they found a used transmission and are rebuilding it for me. How could the transmission case get cracked? And how could the fluid have been full if there was a crack? Thank you!!!

Edit: only 70,000 miles. 🙁