Ponies evolved partially bipedal characteristics as a way to get to fruit trees as an outgrowth of rearing. Ponies who had bipedal behaviors had an evolutionary advantage due to having access to a novel source of nutrition and eventually partial bipedalism became a dominant trait.

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But...
Ponies buck fruit trees. They don't reach for the fruit at all.

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6453078 She's still bucking the tree, just with forelegs.

Ponies don't seem very bipedal to me. I've seen dogs more bipedal than ponies.

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Just look at the expression on Applejack's face. Pinkie Pie is the worst example of apple (cherry) bucking one could possibly find.

Wouldn't this only explain earth pony bipedalism?

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6453214
Indeed. If this were the case, why don't we tend to see pegasi fruit/vegetable harvesters? If merely raising your body up a bit to gain a few inches of height was an advantage in food gathering, all earth ponies would be at a severe disadvantage compared to pegasi who can reach the entirety of every plant to gather from.

Kicking trees seems to be a fairly uniquely Apple family thing. We saw in The Perfect Pear that Granny Smith and Bright Macintosh tended to be the only ones working Sweet Apple Acres, while Grandpear had a lot more workers, each using ladders and things to climb up into the trees to harvest pears.

Tool use and the associated brain evolution preceding it would be the advantage that helped the most. Semi-bipedal activities that free up the front hooves to use tools is not so much an evolutionary step but simply learned behavior. Again, evolved brain capable of learning rather than less sophisticated passage down of behavior via genetics.

If something as simple as evolved behavior/instinct were at play, you'd see mostly a lot of the same harvesting strategies. You'd see selective pressures for especially tall, long-limbed earth ponies. Instead, we just see ponies adopting different strategies and making use of their body innovatively.

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6453043

Speaking of "evolutionary steps" I'd expect everypony to be an Alicorn or at least a unicorn by this logic. I mean come on! Telekinesis? That's an almost unbeatable advantage in more than just fruit gathering.

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Agreed. Considering that ponies directly control their environment rather than evolve in response to it also seems to indicate that evolutionary processes are slowed or halted in them. Disadvantaged ponies still reproduce due to social communities and presumed support networks. Earth ponies, in particular, are depicted even from pre-unification days as producing so much surplus food they can entirely support two other tribes that specialize in other areas.

As with humans, learning and cooperation allowed them to throw off the pressures of natural selection. We'd have to roll back to a time before ponies developed those traits and try to figure out what "proto ponies" looked like and how they behaved in order to address something as environmentally driven as a mere tiny height advantage-behavior.