Why would this have happened anyway? Sorry
Because at the moment, every lorry drives off the ferry (or Eurostar train) and straight on to the motorway and because the UK and EU are in a single market and customs union, there are only spot checks carried out on the arriving vehicles, both on the UK side of the Channel and on the EU side (at Calais, Dunkirk, Zeebrugge, Hook of Holland, Rotterdam, Esbjerg etc.)
On 31st December the UK leaves the SM and CU so every lorry has to be checked and papers need to be stamped. This is regardless of whether there is a Deal or No Deal.
The time taken to check the vehicles (at either or both ends of the journey) means that the ferry and train capacity will be severely reduced, causing a bottleneck and resulting in tailbacks on both sides of the Channel. The logistics companies and RHA estimated that a check of just one minute per lorry would result in a 17 mile queue. Many loads will require checks that take 15-20 minutes, and some might take over an hour.
Gove refused to engage with the logistics industry and refused to accept that there would be the need for overflow lorry parks. As late as June he was still denying that these would be needed - before the government finally realised that the lorry parks would indeed be needed, at which point they purchased land at Ashford (and elsewhere) for the parks. These were only supposed to be holding areas, so little provision has been made for toilets, showers, food, internet connections, fuel etc and the Ashford site is not actually ready - which is why 3,000 lorries are parked up on an airfield with no facilities.
The event that triggered the chaos was the French government deciding to halt all traffic from the UK.
The inability to deal with the chaos (which might equally have been caused by winter weather, fog in the Channel, industrial action in France or the UK, or by trucks breaking down near the harbour, or ferries breaking down etc.) lies with the UK government and with Gove, whose great plans were shown to be completely inadequate when faced with reality.