Any payment funded by your bank account will be sent as an "echeck" if you don't have a valid credit or debit card attached to your Paypal account. If you've always made "instant" payments in the past but still paid through your bank account, it was because you had a credit or debit card attached to your account as a backup funding source.

Most likely the credit card you had on file has expired or you deleted that card. Log in to your Paypal account and select "Profile" - under "Financial Information" select "Credit Cards" - if it is an expired card then you will want to EDIT the card on file and change the expiration date. If there is no credit card listed, you'll need to add a credit or debit card to your account.
All bank account payments are technically "echecks" - that require time to clear, just like a paper check but by having a credit card on file PayPal knows they can bill the credit card if the echeck payment bounces - so they call the "instant" payment.