I'm glad it worked for you, and you're asking a good question. If there's some kind of problem with the drive or Windows detects some kind of errors going on, it may itself set the drive back to PIO mode. So, you'll have to keep an eye on it. I'm not saying there's any problems with it, mind you. But that's one reason why it could have been in PIO.
You should take a look at your "Event Viewer". Go to Control Panel, click on Administrative Tools, and then click on Event Viewer. Look in the System and Application logs; primarily System in this case. You'll be looking for Red (error) and Yellow (warnings) icons before the events. Go back in time and see if you see anything that looks like it had to do with DMA transfers or the disk storage system or similar, if you notice these red/yellow icons. You can report what you see back here.
You should take a look at your "Event Viewer". Go to Control Panel, click on Administrative Tools, and then click on Event Viewer. Look in the System and Application logs; primarily System in this case. You'll be looking for Red (error) and Yellow (warnings) icons before the events. Go back in time and see if you see anything that looks like it had to do with DMA transfers or the disk storage system or similar, if you notice these red/yellow icons. You can report what you see back here.