Why are journals telling me that my article is plagiarized even though I wrote it by myself? Recently, I wrote an article, and I'm trying to submit it somewhere, but two journals told me that my article is plagiarized even up to 70%! 


I'm 100% sure that it's not plagiarized because I wrote it by myself.
The only thing that I suspect caused this problem is that I posted the pre-print online in ECSarxiv, and I think they found my pre-print and think my paper is plagiarized! My name on the pre-print is exactly the same as in the submitted paper.
I checked the pre-print policy of these journals, and they clearly stated that they don't have a problem with pre-prints.
I checked it with Turnitin, and it says there is no plagiarism in my article. I also searched the Internet for random sentences from my article and it just shows my pre-print.
I complained after I received the rejection, pointed out to my pre-print, and asked them to name the paper I allegedly plagiarized from. However, they did not respond back to me. I now sent an email to editor-in-chief and I'm waiting for response.
I am confident that the journals are reputable (Elsevier and Springer).


Any ideas or suggestions?
Chances are the journal ran your paper through an automated plagiarism checker (the only realistic way to check for plagiarism these days) and this 70% number is what the program found. That you found nothing using Turnitin isn't enough evidence to prove there was no plagiarism: a plagiarism checker is only as good as the data it has, and it's possible Turnitin doesn't have the original paper.

Having said that the journal should tell you which paper they think you've plagiarized from, as well as which sentences are plagiarized. If they don't, you should absolutely write back to ask. From your comments you've already done that, so there's nothing to do now except wait. The fact that at least two journals have checked your paper and found plagiarism is a bad sign; on the bright side, you can ask both journals for more details and it's less likely they both don't answer.

If they respond you'll be able to fix the plagiarism if it's there, or point out why there's no plagiarism if it isn't there. 
If they don't respond after a reasonable time, then the only thing left to do is submit the paper elsewhere.