I've seen this before - hard drive boot priorities changing in the bios by themselves. Sometimes caused by plugging in a USB device or another drive which can be bootable. Once you've reset the order after adding a new drive/device and saved, the bios should remember it.

Not sure what causes it otherwise, perhaps some bug with SATA or AHCI and hot swap capability. It is quite rare however, so keep an eye on it. Just seems to spring up now and again.

SATA has caused a few issue with me, I think I've mentioned it in the past. Not as robust as good old IDE and could have been designed much better. The cables can easily go bad, make poor contact, don't like bends and get knocked out of sockets if no latches.

Also I'm finding modern SATA disks have quality control issues - the high density surfaces are of varying quality (more common to see reallocated sectors, many more slow sectors exist today). The same drive models can have different performance characteristics, surface quality, access times and acoustics (varying whining, vibration, seek noise). Had this problem with Samsung, WD and Seagate. Two identical drives having good and bad examples but passing for retail, probably because most people wouldn't notice unless they ran tests. It's becoming luck of the draw!