Not a gun but enormous pressure, after 3 years of May's messing about, only ending up with that so-called "Chequers Agreement" failure. In Boris's haste to pick up the gauntlet and get Brexit done, he only succeeded in minor changes to that agreement - that was all done to quickly in the end but it is one reason why Brexit is such a mess, even though, technically, we are out. Yes, responsibility lies with UK governments - not only the recent ones but also the Heath regime of 1972 and after that - if there had been referendums held then, and before each major treaty signing, then we would either have never joined in the first place or been out long ago.
There is no requirement for a withdrawal agreement but after so long as a member, then a sensible one is obviously best, especially w.r.t. all EU and UK citizens rights and status. Article 50 of the Lisbon Treaty is worded so simply and briefly, that anyone reading it might assume that it would be easy to leave the EU but these words are rather meaningless to any member trying to extract itself from the rats maze that the EU has become after successive treaties, over decades. The EU reaction after the referendum revealed its fear that the UK leaving might mean the disintegration of the EU itself. by prompting others to follow suit. The post referendum vengeful behaviour of the EU commissioners has now been seen and noted around the continent and the world - they would have been far better off being more reasonable and, using their own words, acted in "good faith".