Arch Linux - Read only filesystem after a small period after boot

It's odd.. This issue has never affected me before recently.. I can boot just fine, I get into plasma desktop, but after a period of time my filesystem goes read only..

What would be causing this?

My fstab:

# /dev/sdb2
UUID=211d5868-1b92-4beb-a4cf-135c2a8bdf99	/         	btrfs     	rw,noatime,discard,ssd,compress=lzo,space_cache	0 0

# /dev/sdb1
UUID=9C2D-BF05      	/boot/efi 	vfat      	rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro	0 2

# /dev/sdb3
UUID=1bcd4167-1cf6-495b-af5d-4e5bfecdbb27	none      	swap      	defaults  	0 0

It might tell you in the log (dmesg, or whatever you have kernel logging set for in syslog) or in one of the TTYs (Ctrl+Alt+F1).

Log might be a bit ambitious because it might have gone RO before the line got written, but who knows.