The most likely cause is underexposure on the second frame. One answer notes the second image appears to have deeper depth of field, which suggests you stopped down the aperture. If you made the aperture smaller but didn’t compensate with a slower shutter speed, the negative would be underexposed.

With film, labs or automated printers often try to brighten an underexposed negative during scanning/printing. That can make the photo look flat, gray, low-contrast, and grainier than normal. That matches what you’re seeing.

A less likely possibility mentioned is flare or stray light reducing contrast, but the stronger consensus is underexposure caused by a setting change between shots.

So the likely mistake was changing aperture or shutter by a stop or two between frames. If your camera is fully manual, check whether the aperture ring or shutter speed was moved accidentally. If it’s automatic, a metering or camera-setting issue is possible, but the result still points to insufficient exposure on that frame.