This is more likely a camera readout/electronics problem than a copy/paste or card-speed issue. Because the sensor is read line by line, simple narrow horizontal black lines point toward a fault in the sensor readout chain or related electronics (such as the ADC), not normal shooting speed limits.

If a memory card or file transfer were corrupting RAW files, you’d usually expect broader corruption, blocky/boxed artifacts, or whole-file errors rather than clean linear defects. A permanently bad sensor row/column would also tend to appear in the same place every time; if the lines appear randomly, interference or an intermittent camera fault is more likely.

So the most likely causes are:

• intermittent sensor/readout electronics fault in the camera
• possible electromagnetic interference, though that’s less common unless there’s a nearby unshielded source

If it keeps happening, the camera should be checked/repaired. To help confirm the cause, test with a different card and shoot a series of frames under controlled conditions; if the lines still appear, that strongly points to the camera rather than the card.