Meowy Catkin
Meow!
Absolutely this.
Having had a good number of working stallions, I tend to get grumpy when I hear of a gelding having 'stallion like behaviour'. Nope, its being a bloody rude/agressive thug.
I agree too and that's exactly why I said the behaviour may not be related to hormones.
TBH my inkling based on the OP's posts is that it's probably a combination of new yard, new owner (and this is their first horse), plus the change to individual turnout.
I'm probably wrong though as I've not seen the horse and there are other possible things that are a factor (saddle fit, change of feed?). Hopefully the OP will be able to work out what's going on and take measures to have no repeats of the behaviour.