exfat on SD card getting mounted read-only

[jd1008]

My flash card slot in the laptop is (per lspci):
03:01.2 SD Host controller: Ricoh Co Ltd R5C822 SD/SDIO/MMC/MS/MSPro 
Host Adapter (rev 21)

I insert into it a SanDisk 16GB Extreme SD card. This card is not marked 
as a secure card.
At least, it does not say so on the card.

It gets automounted read only as:

/dev/mmcblk0p1 on /run/media/jd/3D90-BEAB type fuseblk 
(ro,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other,blksize=4096)

# umount -f /dev/mmcblk0p1

# /sbin/mount.exfat -o rw /dev/mmcblk0p1 /mmcblk0p1
FUSE exfat 1.0.1
WARN: `/dev/mmcblk0p1' is write-protected, mounting read-only.

So, what is causing it to be mountable read-only, and not get mounted 
read/write?

PS: If I insert the card into a USB flash card adapter, I can then mount 
it RW.