sourness is from lactobacillus or pediococcus, the former souring wort in a few days and the latter needing up to a year to get a decent sourness, so we can probably rule that out. Most strains of lacto are hop intolerant and won't be able to sour wort beyond ~10 IBU, and even with less IBU it will severely limit what it can do. Where my memory isn't so good, lacto eats proteins as well as sugar, so maybe there aren't the right proteins in apples?
It's a similar story with certain brett strains as well. Brett is hop tolerant and depending on the strain gives off a wide range of flavours, including a tartness, although this is a lot more restrained than the sourness lacto can give. Similarly it eats proteins and sugars brewers yeast can't eat and maybe these aren't in apples? Having said that I have had brett get into batches of cider but nothing sour or tart.
It could be the yeast is ok but somewhere along the line a bug is getting in due to sanitisation issues.
If it's tasty then that's great and enjoy! I love sour beers. Apparently Berliner weisse is often enjoyed with fruit syrups to balance out the sourness.