Do symptoms occur, not happen?

I showed what my friend wrote to an American.

'Physical toll of flying long hours made my hair and skin weaker than before
and symptoms of Intoe happend because of in-flight shoes.'

And this is her version of correcting it.

'Physical toll of flying long hours made my hair and skin weaker than before
and symptoms of Intoe happened -> OCCURRED because of in-flight shoes.'

Well, as she was runing out of time, she didn't correct every mistake found in the sentence, but big ones, I guess.

Anway, from her correction, I concluded that 'symptoms' usually don't go with 'happen' but 'occur'.
To confirm my guess, I looked it up on the net.

To my surprise, however, I got to find several sentence which include both 'symptoms' and 'happen', like,

'Do pregnancy symtpoms happen two days after sex?'
( http://qr.ae/TUpOQw )

'When do seroconversion symptoms happen?'
( http://www.thebody.com/h/when-do-seroconversion-symptoms-happen.html )

Well, now that I've witnessed several phrases using both 'symptoms' and 'happen', I can't help but wonder why she corrected it that way.
If she were with me, I'd ask her right away, but she's left for the US.

Do most native speakers believe that symptoms occur, not happen?
I'd really appreciate your comment.
Thank you in advance.