It's a trend.
It's not just Lily, there's Lilia, Lila, Isla, Lola, Lyra, Layla, ... Liliana.
Other trends are the Eva names - Eve, Evie, Ava, Adea, Ida,Evelyn, Edie...Olivia.
The Ellie names, Elle, Ella, Bella, ...-elle, -ella.
The A names - Aurelia, Aurora, Arabella, ...
Lots of Ls, Vs and vowels.
Boy's names popular seem to be surnames, biblical names or anything ending in o. Again, lots of vowels.
When I was at school, Ann(e) or Jane were the popular middle names, and lots of names ended in -anne (Joanne, Suzanne, Leanne..).