Windows 7 64-bit, GTX 770 OC
Blender 2.78.5 5c96312
I assumed this was just a weird way of how the NLA works but I suppose it won't hurt to report in case it's a bug:
While an action is active, below a transition clip, it the active values will multiply (or something, it's far too much for a regular mix blend). The offshoot only occurs for any animation data shared between the 2/3 transitioning clips and the sub-layer's clip(s).
In this example file,the first 3 tracks are what's expected, and the other 4 (at the transition clip) are what happens that I hope is a bug.