Re popularity trends, I think a particular name or type of name often starts the ball rolling - either via a celebrity baby, or something in the media, or just because it somehow captures the popular imagination - then when that name starts getting popular people look for alternatives that sound similar.
For example, people who grew up with/liked the name Emma in the past might pick Emily for a DD, then other people who like Emily but want to be different might go for Amelia instead. Then when Amelia starts to become ubiquitous people start looking for more alternatives - e.g. a foreign twist like Amelie, or a tweaked spelling like Emilia. I reckon Elodie's current rise in popularity is probably down to a combination of people choosing it who would otherwise have gone for Amelie, plus the fact it contains the currently trendy 'El' sound.
Recently the fashion has been for light, informal, very feminine-sounding names and lots of e and l sounds - hence Lily, Tilly, Milly, Lola, Lila, Lyra, Isla, Ella, Ellie et al. The other fashions at the moment seems to be longer, Latinate names ending in -a, e.g. Sophia, Isabella, Francesca, Amelia, and 'granny chic'/vintage names like Ivy, Ruby, Nancy, Violet and Elsie (which also nails the El- trend).
Agree with Jenni2legs about the middle name thing - looking at recent school registers it seems to be Rose, Grace or May (plus a sprinkling of Elizabeths and Catherines) for almost every primary-aged girl. (Was Louise in my day, which probably dates me pretty effectively!)