Get behind on

Failed to pay.

Thanks a lot for your reply, deadrat.

Wouldn't "get behind with" be a correct form?

No, the person became indebted to his landlord, i.e., failed to pay the rent, in the past. So "got," the past form of "to get" is used.

Thanks again, deadrat.

http://www.oxfordlearnersdictionaries.com/us/definition/english/get-behind
get behind (with something)
to fail to make enough progress or to produce something at the right tim

Yes, indeed.

Thank you, deadrat.

I wonder why the dictionary doesn't list the phrasal verb "get behind on."

Do "get behind with something" mean the same as "fall behind with something"?

http://www.oxfordlearnersdictionaries.com/us/definition/english/fall-behind-with
fall behind with something

You fall or get behind on a series of periodic obligations -- credit card payments, rent, mortgage payments, and as you found, school assignments. Note that generally these are all things that you can "catch up on," by making good on the late payments. You are then "current."

Thank you so much, deadrat.

So, is there no difference between fall behind with/on and get behind on/with?

No difference.