Even though his step-father didn't die until 1939, by 1931 Thomas would have been much bigger and stronger, and he may have felt confident in being able to "stand up" to his step-father.
I imagine Thomas' time in Australia was a great adventure that most young men would have enjoyed - working in the outdoors along side mates of the same background, and learning all sorts of new skills that would serve him well later in life, while growing in confidence, resilience and self-reliance.
Added
Oxcat1, I just had a look at the photo on your ancestry tree of Thomas as a young man - he was a very good-looking chap.