Hydraulic fluid in a reservoir is difficult to ignite. The same fluid escaping a pinhole failure in a line operating at two or three thousand PSI is a different substance in fire terms: a finely atomized, aerosolized spray with enormous surface area relative to its mass, capable of igniting from a hot surface that would never light the bulk liquid. This is why hydraulic equipment fires are disproportionately severe relative to how inert the fluid seems.
The hot surface is usually already present. Presses and forming equipment run adjacent to heated dies, hot workpieces, motors, and in many shops immediately alongside welding or grinding operations. An atomized fluid stream from a failed hose or fitting only has to find one of them. The failure itself is frequently a fatigue crack or an abraded hose at a flex point, which gives essentially no warning and produces a spray rather than a drip.
Once ignited, the pressurized source keeps feeding the fire. Unlike a pool fire that consumes a fixed quantity, a hydraulic spray fire continues at full intensity until the pump stops or the reservoir empties, and the reservoir on a large press can hold several hundred gallons. This makes automatic pump shutdown on discharge as important as the agent itself, and it is the interlock most often found missing or untested.