After having a glance at the DMC of the candidate, one can easily rule out the incapability or poor writing skills. However, still there can can be many reasons behind his/her failure:

Candidate couldn't comprehend the essence of the Topic (likely)

He was unaware of the structure of the essay (unlikely)

Something in writing or technique offended checker(likely)

Checker was having a bad day (likely)

Checker couldn't understand his/her dense expression (likely)

Checker was high at the time of checking ( most likely)

PS: This is coming from someone who had similar DMC in 2013