I have not done this in ages, but if I remember correctly it is because paypal code does not use javascript to handle the button click. And Since the whole Dnn website is a form, then pressing that button posts back to your site instead of to paypal.
Workaround:
As we cannot nest forms, in your paypal code, you need to remove the opening and closing form tags.
Then change the button so that it modifies the form post url and then submits the form instead of just submitting normally. In your button, create an onClick property like this
onClick="this.form.action='https//
www.paypal.com/.......';this.form.submit();"
replace ...... by whatever was in the original form tag for the url (I do not rememeber the specifics.
Note: This is what I remember from many years ago, not sure if paypal forms are still like that, if not, give us a code sample please.