How about making it into a question:

Ben kimin o şeyi yaptığını neden şimdi anladım?

This could be a line from a detective to his team in a TV show:

Why do I now understand who did that thing?, or more colloquially (How come I now understand who dunnit?

PS I wouldn't have got this close without the clues from taksim and aslı above!

I don't think that making this interrogative works, because you would then need the 'mı," as in "Ben.... anladım, mı?"

I forget the mI all the time while trying to speak, BTW.

No. Neden is a question in itself.

Q words:
Niçin
Neden
Ne
Nasıl
Nerede
Nereye
Kim
mı (and all other forms with vowel harmony).

e.g. neden geldi? Why did he come?
Nereye gittin? Where did you go to? do not need mi with them to make them a Q.

Sorry, forgot Kaç as another Q word!!

Also ne zaman
nereden

Well Marrione,

It would be better if we had a native speaker to weigh in on this, but I think that while neden can be a question word - and you're right, when so used, it isn't accompanied by "mı," - it's not a question here: it's the direct object.

Think of it in English:

"Why did you come?" is a question.

"I now understand why you came" is not.

Clear as mud?

It is like "because of"
Buraya gelme nedenim seni özlememdi.
I came because of ı missed you
Or the reason ı cme here ı missed you

Neden: reason cause
neden: why

BTW. these are not coming from some dictionary. Just me.

Even though "neden" is usually used instead of "...yüzünden", "dolayısıyla", "bu bakımdan", "bunun için", "sebebiyle", "verb base + -diğinden" by many people it is a wrong use. It is a question word.

For example: It would be wrong to say:

"Havanın güzel olması nedeniyle pikniğe gittik"

We must say:

Hava güzel olduğundan (= verb base + -diğinden) pikniğe gittik.

or

Hava güzel olduğu için pikniğe gittik.