Twilight’s the only one who didn’t get to see it appear.

@Sjogre  
I think the best thing the show has done for Cutie Marks is having Cheerilee - the teacher no less - explain them and how she interprets hers. At a glance, her Cutie Mark looks like something for gardening, but to Cheerilee, it is a metaphor for her desire to help young ponies grow.
 
To this day, I subscribe to that explanation in spite of other theories, it just makes it simpler and poetic.

@Sjogre  
So what you’re saying is:  

@Fortune  
Except Applejack is perfectly capable of farming elsewhere, she just doesn’t want to. She’s staying with her family, and the orchard is part of her family history, but if her family moves, so will she.
 
The thing to remember about cutie marks is that they reflect what the ponies like, but that doesn’t mean that it is going to be their job. The most obvious example of that is Rainbow Dash, who has never treated her job as anything more than a stepping stone to the Wonderbolts, and usually less than that. Likewise, Pinkie’s primary job is working at a bakery, rather than being a professional clown or the like.
 
Ponies can and do get jobs that aren’t related to their marks. They aren’t forced into anything by them.
 
Also? The designs are abstract. They only mean what the pony says that they mean.

@Sjogre  
For the most part in regards to the Mane 6, their Cutie Marks are vague enough so that they can do anything that somewhat falls within what caused them to show up in the first in the first place.  
Except for Applejack. Her’s says “I’m going to stay at Sweet Apple Acres forever”. Twilight’s is just “Magic in general”, Dash’s is speed, Pinkie’s is causing happiness, Rarity’s is bringing out beauty, and Fluttershy’s is showing kindness and care.

@Fortune  
Except that all of them have stepped away from what their butt pictures symbolize without issue… And they decided what their tramp stamps would look like in the first place.

@Fortune  
Because it represents what you’re best at and love doing the most Destiny.

That magical moment when a tattoo on your ass more or less tells you what you’re going to be doing for the rest of your life.

At that moment, Rainbow Dash realized she loved going fast was Destined to do weather for the rest of her life.