1968-D LMC Weak Mint Mark - Cause?

Some mint marks from that era are punched very hard and deep, while others were lighter.
Since the mint marks were punched by hand, the depth varied from die to die, some were even punched at an angle, some slightly sideways. It is probably the way the mint mark was punched.
It could have been partially polished away from the die also.

EDIT: After looking closer at your coin, it appears that it was most likely over polished in that area. See how the details of Lincoln's coat are also light? There are abrasion marks in that area too.

I'd be with over polishing.

The mint technician didn't assign a right and left security, no pow teams, no demo teams, no line of advance, no rally or alternate rally point, no TRP's, and one of his helpers got caught dipping Copenhagen. All leadership slots received nogo's and were recycled back to phase one at Fort Banning.

SF11dude I been wanting to say welcome and what's up so what's up? OK now to the meat and bones...come out with some goods to the game, your 68 D is weak man. If your Tabbed
SF I want to see your big guns. Easy fella, us regular army guys just like to get the chicken eatersriled up :) when did you ETS? I got out in 2000 at Riley yes I was a Mechanized puke but loved every minute.
Check this out, go to John Wexlers site and buy his and Kevin Flynns book " The Authoritative Reference of Lincoln Cents" its the best out there and he will autograph it for you, get the cherrypickers handbook also as it is a handy reference. The authoritative book has over 1000 photos and just about everything known about Old Abe.
If you already know all this than break out a big one. Remember, I'm only giving you rash like the moderators do me :) Keep hunting and posting glad your on the forum, a belated welcome.

Found one of these in my collection too . the thing that caught my attention was that the "D" also seemed to be in a lower location than the usual placement.