It works like this...........
They sent you a letter , requesting medical documentation of your disability claim. Obviously because you had not notified them why you were absent in the form of writing.
Since you had not notified them, and they sent you several letters of request for documentation , without you returning anything.
They by law, can consider it "abandonment of duties".
Basically not turning up to work, without notifying the employer. (lack of medical documentation)
Since you sent the medical documentation AFTER several attempts to contact you. It could be too little too late, on your part. Thus makes the forced resignation legal.
It comes down to timeline.......................
The dates, on the letters they sent you
Other attempts by them to contact you about the issue with no response
And the date on the medical documentation you submitted after finally meeting their request to send it.
Depending on laws in your area, being absent for a certain period WITHOUT notification can be considered abandonment of duties.
Its not enough, that you call your employer and tell them you are claiming disability. Voice calls and voice notification are NOT documented. It has to be backed up by medical documentation , provided to the employer by a medical practioner/doctor to prove your claim.
Otherwise they can have reasonable belief, that your simply lazy and want a few days off, thus consider it legally "abandonment of duties". Especially after several attempts to contact you for the documentation.
Edit: Timely manner means before they request it, not after.