Delays because of increased French security checks (and not enough manpower), slowing things down. Looks like a deliberate 'go slow'. As if "you've been lucky to have had relatively free movement, here's a taste of what you'll get".
A little tale: I remember in.. Very early '90s... Could have been part of the Masstrricht Treaty discussions in 1992, but I'd check the date before I made that definite, the UK were specifically opting out of (or continuing to) a border-free arrangement, demanding (as we always have) that passports were still needed to enter the UK. I was on a trip around the alps, and as well as the Channel crossing did France/Italy, Italy/Switzerland and Switzerland/France (also, pre-Euro, had to take French Francs, Lire and Swiss Francs with me, which was fun for budgeting purposes). Some of us crossed some of the borders by mountain passes, but those who drove round by road-borders in the British registered minibus were always stopped for passport checks at each opportunity, whilst local (or even non-local but European) vehicles were let through without note. It was clearly 'unofficial policy' to hyper-enforce the borders to obvious UK Nationals.
Like now, there was no known threat from British travellers. All the problems that anyone, eurosceptic or europhilic, could ascribe to Europe from 'foreigners' (who often aren't even that) in Paris, Brussels, Nice, Munich, etc, there's nothing known (except for briefly the 'traditional' anti-hooligan programme surrounding the European Cup, which is obviously not a current concern) that would require French borders to be made more vigorous w.r.t. UK visitors.
Not saying it is that (there might be specific intelligence, not on public release, being acted upon), but it does look a lot like "today, my department will 'happen' to be overofficious with Les Rosbif..." doesn't need to have been ordered from above (Mitterand, or whoever it is now... I'm slightly forgetful of French leaders' names1) or sanctioned or even tacitly approved.
'Operational concerns' may have been invoked whilst beaurocracy finds a way to de-escalate things after the likely complaints from Whitehall, or perhaps kind mention the costs the French have already incurred in buffing French/UK 'borders' at Calais. But I think it's an obvious put-up job. Without evidence, just historic experience, perhaps tinted with mild paranoia and a heavy dose of Schadenfreude (I'm away from home, myself, but not cross-channel-wise).
(By the way, the licence few is the worst funding method except for all the others! I don't like what successive governments have been doing to the BBC. If there's any controversy, it lies somewhere betwixt my view and yours, I think.)
1 I'm rather waiting for Le Pen and Clinton joining May and Merkel and Ang Sang Syu Kui and the several other (potential) female leaders I might half remember in a world-matriarchy. But Marine aint yet there.