personally (and i am speaking from severe personal cock up point of view also… :'( ) i think it is inability to understand and read and manage the dog, inconsistency, harsh handling sometimes alternated with spoiling, lack of training – and if people fail to be able to read one dog then get another – they are going to do exactly the same – and on it goes…. :'(
dorain was attacked by a beardie once – along the canal bank – wasn’t human aggressive though, and was the only aggressive beardie i have ever met. 😉