Another tiresome day “fishing” you know?
Why...? maybe Sammy thought his bro was mimicking him. Or he felt challenged. Or saw a random opportunity to give him a fruit to eat. It's good for his health. But all in all yawning is contagious, simple as that
Also one of the sources for these two's dhynamics is videos of funny animals. Which – on this topic, some people asked why Sammy hates his little bro so much. He doesn't, he probably just expresses affection in more of a cavebeast way.
PS when I sketched this, there were still days that made it okay to wear short sleeves, it's been winter already for a few days now; we still have some bright afternoons. I just love autumn.