I have two guesses. The first would be that the sensor is saturating. The second is that the metal oxide's conductivity is changing as more current is running through it, possibly due to localized spots that are warming.

https://wisense.wordpress.com/2015/07/06/gas-leakage-module-using-the-mq-5-gas-sensor/

It seems that the current used by the mq5 sensor is too large when the sensor reaches its maximum value, there is a solution to overcome this is to add a minimum resistance of 22 Kohm. I hope this helps. Thank you.