- Brake fluids are more viscous when cold
- Water absorption can increase the viscosity
But only until heat causes the water to become gas, which is compressible. When this happens, the lever feels a lot less stiff and pressure at the pad is drastically reduced.
- Your system may have originally been poorly bled.
Tilting and vibration of the bike during use can encourage bubbles to travel upwards. Assuming you have an "open" hydraulic brake system, pressing the lever opens and closes a valve between the pressurised volume in the hose and the free fluid in the lever reservoir. Bubbles at the top are displaced into the reservoir through the valve until it closes, fixing the volume and allowing lever pressure to be transmitted to the caliper. When the lever is released, the valve has another open phase during which (hopefully) fluid is drawn into the volume. This behaviour is easy to observe in designs like Shimano's XTR BL-M975.