It can be several things, mostly it is not having a confident rider and in testing that rider, they find they get out of work.
They soon learn that if they can frighten the rider into dismounting, they finish work.
A rider returning from a ride, feeding the horse straight after and turning it out as soon as it is finished can lead that horse to want to get home as fast as possible get fed and then back out with its friends.
If you child burst into tears and threw a tantrum over having to wash the dishes and you said you would do them, next time you can expect the same reaction from the child, worked last time so why not this.
Some horses just do not have a good work ethic, they are just born with it!
Past can have a lot to do with present, of your mare got away with it with her last owner then she is going to try it on with someone else.
Horses, dogs and children are very sensitive to inner emotions. They react totally differently to someone who is confident and assertive to someone who is unsure.
An experienced rider might get on your mare and ride her with little problem. If she showed the slightest hesitation in going forward they would know and it automatically drive her forward, harder leg, maybe a crack with the whip, and she would know they mean business.
On the other hand it might be such a habit that she would try it on and need very firm corrections.