First, he put some lipstick on Theresa May's withdrawal deal, made it slightly worse, and branded it a "New Deal". Understandably, the Westminster Parliament was not thrilled by this antic and tried at least to ensure that the necessary legislation for an orderly withdrawal was in place. Hence the need for another extension of Brexit.
Secondly, and crucially, in defiance of Parliament Johnson sent a letter to the EU, explaining why they should reject the parliamentary request for an extension.
In a constitutional model based on the sovereignty of parliament, this is not a trivial matter: such an act of defiance is in essence a violation of foundational principles. On top of that this act embarrasses the UK in Europe and the world, because effectively the country does not speak with a single voice. One does not expect from the oldest constitutional parliamentary model such an institutional schizophrenia.
The developments raise a larger point: the relationship between the Sovereignty of Parliament v. the Sovereignty of the People in the UK. Boris Johnson wants to portray himself as a defender of the People v. the Parliament. This is highly combustible political stuff. On the one hand, it puts into question the British constitutional model and calls for its revision. On the other hand, the presupposition that the hard Brexiteers' views are "the will of the people" is wrong: many people voted with the "softest" Brexit in mind in the 2016 referendum. The hard Brexiteers' hijacking of this agenda is actually a deformation of the "will of the people".
So far Brexit had put strains mostly on the unity of the United Kingdom: Scotland was increasingly drifting apart from Westminster. With the latest escalation of the Brexit drama, the very parliamentary setup of the UK is put sharply into question.