Can be a couple things
As you stated wicking not enough causing a gap between coil and wick, excess juice fills the gap, then boils and pops
2nd is in the coil, there is a gap of some sort, as with a wick gap, juice fills this gap then boils and pops
3rd is the wicking is so fast, liquid dances on the hot coil, and spits and pops around
4th the reverse pressure vacuum you cause pulling hard on a sub-ohm setup, pulls a lot of juice into the chamber, the air flow pulls this excess liquid up through the mouth piece
Mostly see this with higher VG liquids at higher outputs, not exact but example 20w plus