What causes what? "Accepted" does not mean you get your refund instantly. When you e-file your return, the first step is getting it accepted into the system. Then it goes through processing at the IRS. The data you entered is analyzed for accuracy---they match it to amounts you entered and compare it to amounts given by employers, check the dependents you entered, check to see if you owe back taxes or child support, etc. etc. etc.
https://ttlc.intuit.com/questions/2840013-does-accepted-mean-my-refund-is-approved
When your return has made it through processing then your refund can be approved and the IRS will give uou a date for when to expect it.