Transmission popped out of first gear

[87 HJ60]

I was driving my HJ60 in first gear down a fairly long steep downhill road today. I wasn't going fast at all, maybe 5 km/h (3mph), then all of a sudden the transmission popped out of first gear and into neutral. My hand wasn't on the shifter so it wasn't me that pulled the shifter out of gear. Is there something wrong with my transmission? It happened once last summer too but when it happened I was driving on a flat section of road. Otherwise the transmission works, shifts fine.
Any ideas?

 

[REKCUT]

my first guess anytime you get movement in the transmisson or transfercase is broken motor mount.

 

[87 HJ60]

The motor mounts are fine. There's 180,000km (110,000 miles) on the vehicle. Everything looks good still.

 

[beaufort-fj60]

mine does it to.. engine twists and the gear lever is "pushed" out of first..

 

[cruiser88]

My 78 40 did it for 50k