Besides external factors, if you look at the top three across each day there's a decent bit of variety underpinned by a backbone of thrash (which is always popular).
Amon Amarth - popular and, whilst not "young", they are a new headliner and have a good live rep, both playing wise and their show.
Blind Guardian - popular, power metal booking and rare outside of London shows
Testament - popular, thrash, tight live
Ghost - on a very quick rise and gaining popularity all the time, quite popular amongst a wider breadth of people than the standard BOA band, have a cool gimmick, tight live
Kreator - very well respected thrash band and not a band that's overbooked at the festival, also got a good live rep
Hatebreed - whether you like them or not, they drew a decent crowd last time and they cover off part of the 'core' crowd
Megadeth - very solid classic band with roots in thrash, rarely has a bad show
Arch Enemy - whilst possibly a smidge high on the bill they do seem to have gathered some momentum again now with Alissa on vox.
Skindred - another "whether you like them or not" they are a damned popular band.
And that's not touching on stuff like finally getting Annihilator, Hell bringing their biggest ever show, etc etc