The explanation is that they're a pair of arseholes. Genuinely.
If I were being charitable - which I'm not you'll have realised - I'd say that they had dependency problems. The aggressive one appears to be an alcoholic; the other is now on methadone. One time - before I made the gate a bit pricklier - I had to replace a pane of glass in the roof of the greenhouse. There was a football shaped hole in it.
The alcoholic really does appear to have a chip on his shoulder. He was ranting to my husband "Maybe you were a hard man at one time, but ye're no noo!"
DH was utterly bewildered at that. He told me "What was he going on about? I've never been a hard man!"
I'm assuming it was a reference to the fact that DH took up karate late in life and got his black belt in his mid 60s. There were no changing facilities at the community hall, so (prior to his stroke) DH would leave the house wearing his karate suit.
I actually get on all right with the neighbour, but her sons are idiots*. When her husband was dying in hospital I felt sorry for her and gave her lifts. The alcoholic had been gifted his dad's car, but it was crushed when he failed to tax it.
The second son is actually quite mild-mannered, except when he's back on hard drugs.
Anyway, I'm a bad person because I've made it very difficult to balance on top of my gate and I've not put up a warning sign.
*Plus I will never forgive the way that the elder of the two spoke to my husband.
ETA Come to think about it, years ago some of the neighbour's washing blew into our garden and she stepped over the low boundary fence to retrieve it instead of knocking on my door. To this day, she doesn't know that I saw her.
I now have a prickly hedge there - mainly holly - which is why her sons couldn't do the same thing.
Maybe they just think that getting the stuff themselves is quicker? Who knows.
Maybe I'm weird, but I just don't like folk being able to stroll into my back garden unannounced.