Oftentimes heroes such as Genji, Tracer or Sombra are the first to mind when considering who might be making a sneaky entrance from behind a point. And while these flankers are definitely a cause for concern, a well-placed trap from Junkrat can alert a team that something is amiss in the backline.
Players who specialize in these flanking heroes are often on the lookout for traps that might give away their location as well, so they can be tricky to track down. This was not the case, however, when the Reinhardt in Stryderhorse's clip attempted to take the role of flanker into their own hands. Perhaps the Reinhardt was also new to their flanking role as a massive tank, which is why they neglected to see the strategically placed trap in the choke-point.
In the end, Stryderhorse was also likely to be just as surprised as the Reinhardt to find them in their trap, and was able to fend off the terrifying armored flanker without either team being the wiser.