At first sight it does look like something like a hugger possibly working loose has been scraping along the top of the tyre. That level of damage should however be matched by significant wear to the hugger so I presume we can rule that out.
Unfortunately that photo does not show the tread depth accross the full width of the tyre, so it is difficult to be certain what the cause may be.
However normally excessive wear along the centre line of a tyre coupled with what appears to be good tread to the sides is a sign that the tyre has been over inflated. Have you checked you tyre pressure gauge for accuracy against another device - service station air lines are somewhat random? It can also be the outcome of regular long haul motorway runs which will concentrate wear on the tyre centre and result in a rather squared off tyre profile and will lead to nervous handling.
Of course if you are inclined to light up your rear tyre that would do it too
Another unknown is how old was that tyre? There should be a date code somewhere on the sidewall of every tyre. If it is over 6 years old, it is probably time to replace even if it still has decent tread.
Looking at that wear pattern I am surprised the tyre fitter who replaced that tyre did not at least offer some thoughts on what might have caused it.
Obviously he is correct in confirming that the tyre was dead and need replacement. However many professional tyre fitters are incentivised to alert manufacturers if they encounter tyres with potentially worrying faults so they can investigate to see if there is a problem batch. I wonder if they alerted Dunlop - as that tyre shows a significantly worrying wear pattern which does not inspire confidence especially after only 3,500 miles use. On my 675 I used to get 10,000 miles on a set of BT21s and 7,000 on BT16s.