Oak tree advice?

[M.B]

Hi I was wondering if anyone could help me with finding out why the bark at the bottom of a customers oak tree is peeling off and starting to look rotten, but the foliage and dead wood looks sound and standard for and oak tree? Has anyone got any ideas ? 
cheers 

Edited May 26 by M.B

[Rushes]

Has the level of the ground around the tree been raised?

When it has been this is generally what happens.

[M.B]

Hi yes I asked that question but they said they said they not as far as they are aware but the soil is quite light under foot which makes me think that they have built soil up on it 

[Al Duffill]

It does look like the soil level has changed at some point, that was my first impression before reading the replies, so this could rot the bark due to moisture and organisms at work. 
 

How long have your clients been at the said place, maybe it happened before they were there?