Maybe they imagined that it would be an instant ripping out of the tethers and cords connecting the UK to the EU?
That was one option, but I imagine most expected negotiations to drag on for a few weeks before the UK left the EU - certainly not years of negotiations leading to years of transitional periods which lead to years of negotiations for the penultimate transitioning period for the ultimate negotiations of deciding when to actually leave
Not sure where things are as far as the Brexit deal is, but I've heard that the latest main hangup has been how to deal with the North Ireland-Ireland border, namely how to deal with everything that you'd have to deal with in a border crossing because Ireland would still be in the EU and Ireland has major economic connections going through there. I doubt it's THE latest hangup, let alone the only one, but that's just one that I've heard about.
Yeah I saw a lot of snarky reddit shit about that from people who knew little about the situation but still wanted to voice their misinformation. The Ireland-Northern Ireland situation is pretty simple* for the immediate future, the Republic of Ireland is not a member of the schengen area nor is Northern Ireland or the UK. The UK and Ireland have had de facto free movement for the past century of peace time in their own bilateral agreements and cooperation, whilst both have maintained the same hesitance towards entangling one's nation with EU institutions. Ireland does not have land connections to the EU through Northern Ireland, its major connection is to Northern Ireland & the UK itself (which is something I would be intrigued to see change, if the EU built a tunnel connecting Ireland to the UK, Ireland would have a direct land connection to Europe through overland railway and the channel tunnel).
Regarding this, in an unusual display of unanimous sanity, Irish, British and EU leaders have all agreed to maintain the status quo in order to not provoke new troubles.
Green = Common Travel Area
Blue = Schengen Area
Grey = Dudes who use borders when their governments exist
*Thus it is simple for the purposes of travel arrangements. Everything stays the same in that regard