Manual Transmission lock-up

I have a 2003 Toyota MR@ Spryer, 64,000 miles. I was driving 30 MPR and the car just stopped. Couldn’t get it into any gear. The back wheels locked. Couldn’t be moved or pushed off road. A bearing in the transmission (?) shattered said my mechanic and the clutch (the disc) also cracked. I had to have the transmission re-build, the clutch replaced (which I have new just 7 months prior to accident). Why did this happen? Is my mechanic being honest with me? The repairs were $2,400 with a year warranty.

The rear diff seized, so suddenly it wouldn’t move, while on the other end the engine was trying to spin the transmission. I’m surprised the transmission stayed attached the the car and/or the drive shaft didn’t depart this world.