The character are walking together and they just meet after a while. The character on the right says "If the wind blows, it still is nice, but it gotten hot"
To that, the second character adds
"That is because you have been training in the club"?
Is the からだろう here giving out the reason here? Why is それは used?
You have mostly answered your own question here.
Is the からだろう here giving out the reason here?
Yes, your translation is pretty much spot on, it's something like "That is because you have been training in the club". I'm not sure what about それは is confusing here - you could omit a topic and this sentence would still be natural, but it's perfectly natural with an explicit topic as well. The それは is basically just the English That is at the beginning of the sentence, making it explicit that the explanation being provided refers to what was just said.
