Could be anything, I agree with chuck, swap the battery first to see if problem persists, I have had to repair shotty soldering on LiPo packs in the past as well. It could be a bad sensor wire or possibly a bad sensor board on the motor, if battery swap doesn't fix the problem, then I would pull the sensor wire and run it in sensorless mode. I have seen guys at the track damage their sensor board on the very first battery pack after taking a nasty crash. I have made it a point to always check the gap between the rotor and sensor board on every new motor I buy to help prevent damaging a sensor board that may be too close.. I have seen some rotors that were actually rubbing on the sensor board! I like to have at least 0.1mm gap to play it safe. It will be pretty obvious if the sensor board is damaged when you pull it out, but there are sensor testers available fairly cheap:

If you think you might ever get into any stock racing classes then I would invest in a motor analyzer which not only helps you test your sensor, but offers a myriad of other features which can help you tune stock motors for peak performance, I use this analyzer here:
https://www.rcjuice.com/skyrc-brushl...r-checker.html