Further, orcas are culturally separated into many groups, often called ecotypes. Although some of these ecotypes may eventually be separated into separate species, they are currently all the same species. They are able to eat whatever food and they are able to interbreed, except that they don't. These are cultural separations that orcas have created, and they are not biological.
So, certain ecotypes are fish eaters, and very particular fish eaters at that. The Southern Resident orcas in the Pacific Northwest only eat a specific type of salmon. That salmon is in major decline, and thus the orcas that eat them are in decline and for various other reasons as well such as being hunted in the 70s for capture to serve as entertainment. As far as I know, it is unknown why these orcas do not switch their food source. From what I read, these orcas performing the attacks are tuna eating orcas, so they are again fish specialists. They are likely experiencing the same threats to their food source as well.
Other ecotypes, like those that you mentioned about seals, have more diverse diets typically revolving around mammals. In the Pacific Northwest, the Southern Resident (the salmon eaters) and Transients (the mammal eaters) share the same waters but do not eat each others' foods. Since the transients have more diverse sources that are not struggling, they are not struggling like the fish eating ecotypes.
Thus, it's highly unlikely for fish eating orcas to suddenly want to eat orcas. There's multiple reasons. There is the cultural reasons, and there are also the reasons as humans are terrible meals. My own personal suspicions are that orcas are much closer to human level intelligence than anyone recognizes, and so my personal guess would be that these attacks are one of stress and aggression to prevent other animals stealing their food. It is also possible they've been able to reclaim tuna this way as well.
There are stories of orcas in the early 20th century helping whalers, but the way I read it, it was actually the orcas training the humans to help them capture and kill the whales. Orcas are intelligent enough that they understand how things work.
There is footage of this technique in BBC’s Frozen Planet, if anyone is curious to see it.
https://www.youtube.com/watch?v=SBRu3LGceAg
It doesn't seem as full effort as the normal ones, so they could have been training or experimenting, both of which they are known to do. They could have been play and giving a little scare as well.
The increasing migration across the Mediterranean, often in overcrowded, unseaworthy boats, raises the possibility that maybe a pod of orcas has come across a boat that is sinking, putting seal-sized, slow-moving creatures into the water...