Glad to hear you found time to fish those streams!!
I can't imagine someone stocking a 6" tiger trout in Valley. Probably a wild fish then. THe question is how did it happen and I'm pretty baffled on this one.
My initial thought was one of the brookie plants in Crabby Cr a few years back did not find good holding water at adult size and headed downstream, all the way to Valley. Without finding other brookies, it had no choice but to spawn with a brown once fall came around. BUT, there's a huge problem with this theory...for the tiger trout to get upstream to where you caught it, it would have had to go over two large private dams, and I can't see that happening. I've been skeptical of what some call migration barriers on some streams but those two dams are unquestionably too tall to leap.
I think it leaves two possibilities.
1. there could be a tiny remnant brookie population hidden away somewhere in the watershed. It wouldn't be the first time that's been discovered recently in chester county. When they get down to very low numbers, they can be very hard to trace and just barely hang on for much longer than you might expect. Just a handful of adult fish moving in and out of various connected streams and occasionally getting a successful spawn to happen somewhere. It happens in the Gunpowder here in MD.
2. someone planted adult brookies and one spawned with a brown.