I'm guessing that was probably an over-distance event registered as a BRM. BRM rules are that finish times are not calculated by speed, they are simply set times, with no allowance for over-distance. So for example a 300km finish time is set as 20h (==15k/h). If it is actually 310km, the time remains the same but that is now (==15.5k/h), but intermediate controls will still be timed at 15k/h which is the fastest 'minimum' that AUK's rules allow.
So yes, in this instance the final leg can be a bit of a rush for the slowest riders, with (in the example above) 40 minutes lost. The situation is exacerbated if the organiser has set a lower min speed, such as 14.5kph (this shouldn't really happen, but it is possible under AUK's rules).