Why do the 737-100/200’s thrust reversers blow fully open if hydraulic pressure is removed while the reversers are partly open? If the thrust reversers on an Original-series 737 (the 737-100 and -200) are 2 inches or more from the stowed position, and hydraulic pressure is removed from the reverser actuators, the reversers will travel to the fully-extended position under aerodynamic loads at airspeeds down to below 130 KEAS (knots equivalent airspeed), as was discovered during the investigation into the crash of Pacific Western Airlines Flight 314.  To quote the official accident report:

... It was found that the air loads on the reversers were insufficient to deploy them further if they were freed with the leading edges 1 inch out from the closed position. If the initial deflection was increased to 2 inches, they invariably deployed fully under the air loads. At intermediate settings, deployments only occasionally occurred. ...  [p. 17-18 of the report; p. 23-24 of its PDF file]
... The ground runs showed that partially opened thrust reversers deployed at various speeds during taxi tests with manually controlled airlground logic. The rate of deployment varied with the amount of the initial open condition of the reverser doors and also varied with speed. In all cases partially opened doors deployed fully before 130 Kts.  [p. 18/24]

However, the 737 Originals use target-type reversers (you know, the kind with the two buckets that close behind the engine’s tailpipe, the ones that everyone immediately thinks of when they hear the phrase “thrust reverser”); I can easily see how they would be blown to the full-open position if the engine(s) aren’t operating, but, if they are running, then, without hydraulic pressure holding the reverser buckets in position, shouldn’t the engine(s)’ jet blast force the buckets apart and open a path between them for the engine efflux to exit?
If the buckets could be opened by engine thrust, they would be ineffective to reduce speed. By the same token, you want airflow to help them deploy when needed. The controlling force then is the hydraulic actuators. Without hydraulic power it would be very difficult to achieve positive control over the buckets' position.