I have just noticed this in 2 boys, aged 3,4.
The 3 year old used to throw tantrums quite frequently over trivial matters (such as me holding the ball I'm throwing in the right rather than the left hand). He stopped doing this somewhere in the last 3 or 4 months, while the older one started throwing tantrums over similarly minor things.
When the younger one would throw the tantrums, the older one used to quietly watch (kind of in awe or taken aback). He would look at the younger one as if puzzled by the behaviour.
Now the older one has taken on the frequent tantrums, and the younger one looks on in awe.
I'm not sure what's going on here. I'd like to reduce the frequency of the tantrums, but the weird thing to me is the behaviour swapping. Will this continue to happen? (ie tantrums from one but not the other and vice versa, in periods?)