It would also be interesting to see how many levels for each class and at what time people usually decide to multiclass.
I guess there are certain levels that make more sense than others.
Rogue Multis are going to be popular due to how good Thief 3 is. But for this particular combo it's most likely a hold over from table-top where Gloomstalker Assassin is quite powerful. It still is in BG3, but it isn't on the same par as a Thief Swords Bard or any of the other broken combos.
I bet it's range and sneak attack. I play similar in my DnD campaign. I like rogue fighter better in dnd. It was my favorite character, but so far crossbows seem weak in BG3 compared to dnd. I'm only in the 1st area in BG3 so maybe they get better.
Thief is pretty good with most things just for the bonus action alone. But if you are a dual wielding martial that bonus action becomes even better since it can always translate into damage. I haven't played a ranger yet though and I'm not sure if hand crossbows are used frequently with them.
I would guess the order just indicates they started as a fighter, not that they are mainly a fighter. So a 1 level dip into fighter first for heavy armor and a fighting style, then melee warlock from there makes sense to me.
Otherwise, maybe it's just people thinking that fighter 5/warlock 5 gets you just as many attacks as a full fighter, and also fireballs/hex etc
Right now the Pact of the Blade Extra Attack stacks with the Extra Attack gained from Class features and going Eldritch Knight gives access to the Shield spell and some lower lvl spell slots.
Warlock 5/Fighter 5/Paladin 2 and Warlock 5/Paladin 5/Fighter 2 both hit extremely hard. Only Paladin 2/Swords Bard 10 really out performs them for melee burst damage.