I'm always one for agreeing that flagging up potentially fraudulent activity is worth doing, and for that matter I wouldn't be offended if somebody queried something I did which needed explaining.
I'm glad the OP is diligent about this sort of thing, and I hope their thread of private messages has proved useful.
However, I've come across unusual situations involving RTVs before. For me, often in scenarios where I haven't had much time to decide if the use has been valid or not, there have rarely been reasons to refuse. Ticket offices seem more susceptible to various problems involving walk-up customers appearing out of the blue with odd purchase requests, or other questions. I believe some of our esteemed fellow members have mentioned problems with people cashing in RTVs at remote ticket offices before.
Some people will probably think that their employer, if it isn't a TOC, will not bat an eyelid at more obscure transactions because expertise on this sort of thing is lacking.
However, I can think of other possible explanations which might be against Terms and Conditions, but not intended to defraud the train operator through false claims (etc. etc.).
One wonders if this could be a case of two people commuting on two separate season tickets for vaguely the same route (flatmates / husband and wife / etc.), therefore accruing two sets of vouchers each time they are both working and using the trains at the same time. One of them then needs to renew the season, the other doesn't. As a result, the first ticket holder gets given the RTVs to renew, while the second is quite happy with a bank transfer / paying household bill [x] in full for a while / etc. The claims have been legitimately made, it's just that the vouchers have changed hands for what is (from a human point of view) a perfectly normal sort of reason.
A delay on a substantial leisure journey could also account for an unusual spike. Some relatives of mine recently got a legitimate, large Delay Repay refund for using VTEC First Class tickets during a day of absolutely massive disruption, way above what either would usually get for their usual local journeys.
Lastly, I wonder if perhaps some sort of enhanced compensation scheme for major disruption of some description could result in a bulk issue of RTVs with a similar value.
Other scenarios are, of course, also available!