I'd guess that it's one of those DIY jobs on a par with panelling over
those 'nasty' Victorian doors - and has probably been done in the
simplest way possible (which would be to leave the existing uprights
in place and simply fix the ply (or more likely, hardboard) to them...

That would be my guess too; you might find one or two broken which might be
why it was panelled, but they are likely to be square and therefore
relatively easily replaced.