It's all due to atmospheric conditions, as has been explained before. There can be many; there can be none.
There are two main reasons why some planes leave trails and some nearby planes do not. The less common reason, is that different planes have different engines. Some engines will leave a contrail in air where another engine will not.
Secondly, when the plane is in wet air, it makes a contrail. In dry air it does not.
You really should look at the science of all this, and not the junk. Go here for a good explanation.
Why do some planes leave long trails, but others don't? - Contrail Science » Contrail Science