Interesting, the WTT for 3I96 has it sitting at one of the stops between Mitcham Junction and Sutton for 10 minutes, arriving into Platform 2 at Sutton just after the 0758 departs, and then departing as 2W96 to London Bridge via Streatham as it does now (but from P2 instead of P1).
Just speculating here, but considering it doesn't appear to be a permanent alteration, could there possibly be an issue preventing use of the crossover from the down to up line outside Sutton (given it is the only service normally timetabled to use this crossover), meaning it needs to start from Platform 1, which would explain the need for it to temporarily run down to Epsom to turn around. Platform 4 is also occupied during this time so the use of P4 wouldn't be an option either.
Edit - A better use of 3I96 might be to send it around the Wimbledon loop (it wouldn't conflict with anything at Streatham Junction or on the single line through Wimbledon), and possibly start 2W96 further back at Sutton Common, which I imagine would be a useful extra service for Sutton Common and West Sutton! It wouldn't be able to start at Tooting or Wimbledon and serve all stops as it wouldn't be able to get back to Sutton in time to meet the path of 2W96, although maybe an option is to have it start at Wimbledon and run non-stop to Sutton Common, which would provide a potentially very useful fast service from Wimbledon to Sutton right in the middle of the peak time, and could be particularly useful for some of the many schools in the area.