how come form is submiting automaticly when i am cliking on enter ?

Forms were designed that way (regardless if it's built in Jotform or not). This is something referred to as Implicit Submission as described on HTML5 specifications. This behavior applies to the entire form regardless of what field is used (e.g. regular textboxes, one of our Quick Fields, etc.)

This is important to accessibility, assistive technologies and impaired users that cannot use a mouse at all. There are pages on the Web that are only usable if there is a way to implicitly submit forms, so user agents (browsers) are strongly encouraged to support this.

If you really want to suppress this default behavior of forms, the only way around it with JotForm is to get your form's entire source code and use Javascript to prevent the Enter key from submitting the form.

I found a really good read about this topic on this post in case you're interested to know why it was designed that way. At the latter part of that post will be alternatives on suppressing it (especially the Explicit Prevention part where they gave an example JS code snippet to prevent it).

Hope this clears the confusion and in case you have follow up questions, don't hesitate to ask.