Don't assume that Slater authorized this. Lawyers on retainer do this sort of stuff all the time while make-working billable hours for themselves. One thing I've learned is that you really have to watch what your lawyers are doing or they'll end up charging you $350/hour to publicly embarras you.
If that is the case you have your lawyers on a very sloppy retainer.
Going outside the terms of a retainer is classed as professional misconduct where I'm from. A lawyer should not take any action on your behalf without, at the very least, implicit instructions to do so.
The goal isn't copyright enforcement at all because noone (up to and including David Slater's own lawyers) is sure who does own the copyright.
Rather, it appears to be an attempt at preserving David Slater's revenue stream by paying lawyers to rattle a few sabers.