Here are the two versions, numbered for convenience and with sentence-punctuation disregarded:
- You're reading this because you forgot your smartphone when you went to the toilet didn't you
- You're reading this because you forgot your smartphone when you went to the toilet aren't you
In sentence 2, aren't you matches you're reading, which we can write as you are reading. There's no disagreement with this form, so let's focus on sentence 1.
In sentence 1, didn't you matches you forgot, which we can write as you did forget. (It could conceivably match you went as well, but that just compounds the problem.) So the structure of sentence 1 may be thought of as
- You're reading this because (reason)
However, "you forgot your smartphone when you went to the toilet didn't you" is a rhetorical question, not really a reason. So if we punctuate #1 in the straightforward manner, it's not grammatically correct:
- (*) You're reading this because you forgot your smartphone when you went to the toilet, didn't you?
If we punctuate it differently, however, we can argue that the sentence employs anacoluthon:
- You're reading this because ... you forgot your smartphone when you went to the toilet, didn't you?
An anacoluthon is an unexpected discontinuity in the expression of ideas within a sentence, leading to a form of words in which there is logical incoherence of thought. Anacolutha are often sentences interrupted midway, where there is a change in the syntactical structure of the sentence and of intended meaning following the interruption. An example is the Italian proverb "The good stuff – think about it."
- wikipedia
You ask:
Could they both be appropriate?
Yes, they could, but only if the missing punctuation is filled in appropriately.