Not selecting go-around thrust would have to be high up on the list of possible causes.
Regarding the rumour that the FO may have selected F15 uncommanded, I am reminded of various suggestions made on a previous topic of what to do if your captain was determined on continuing with an unsafe approach. It may even have been the one about the Air India Express crash where an experienced Indian FO was paired with a highly experienced european and everyone was killed.
Forcing matters by calling go-around and starting the go-around actions was, if I remember correctly, one idea from certain quarters. Selecting flaps 15 is of course not the first action, but on the 737 it does come after setting go-around thrust and before 'positive climb gear up'. In the best traditions of pprune this is of course all rumour and speculation. But hopefully we will find out reasonably soon what really happened and can learn something useful.