Hello,
Because you have a "form" related to the payment method (in your case, a choice for the payment mode), the system need a button so you can validate the form content.
If you have a payment plugin which require some credit card details, you will have more fields and you would need that button to submit the card details. It's not something which can be submit automatically each time something changed.
Regarding the payment plugin you're using (I don't know which one), you have to know if the action of choosing a "payment mode" will submit or not the data ; but it's the reason of the button.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.