I can't speak on whether or not it's because she's coming into season, but having a 5 year female who likes to hump dogs (if given the opportunity), I've learned that it's all about context.
What's your dog's body language before she humps. Is she giving hard stares at the dog? Are the other dogs being loud, rambunctious, rude? How does the other dog react to the humping? Lila will hump in this situation, but it's not motivated by demonstrating her "rank." She's telling the other dog to take it down a notch. The dog who was humped, doesn't roll down, but either stops what they were doing, takes it down a notch, or will take the play out of Lila's bubble.
Are your dog and the other dog playing? Have they been playing for a while? Are they about to play? Again, what are to dog's body doing? Are they loose and bouncy? She might be excited by the play or trying to initiate play. Does play break out after she humps the dog?
Are the dogs meeting for the first time or coming together for the first time? Is your dog's body stiff? Does the dog who is humped roll down when humped? This might be dominance.
For Lila, there is also the "Hey, you're cute. I want you!" hump. (This is usually reserved for blue eyed dogs or black, female labs.
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There could be a variety of motivators, but the context and reactions of the other dogs will help you begin to pieces it together.
