This usually applies to countries which drive on the right.
Generally speaking (and there are always exceptions), nations which drive on the left (UK, Japan, India, Australia, NZ etc) have the rear brake on the left - to allow them to brake and make cross-traffic hand signals.
Thankfully I'm right-handed, so this works great for me
However, must learn to time my drinks better in future - and not try to hurriedly stuff the bottle back in the cage on a downhill, hairpin descent! Doh!