Estelle seems to have a certain fear of being "trapped", both physically and figuratively to an extent.
This is not to say she's claustrophobic, she likely will have little problem with small spaces provided that there's an exit that is reasonably accessible, and she can leave through it if she pleases. If there is NOT a clear exit, or if there seems to be a high enough likelihood that that exit could be removed in one way or another at some point, she has a problem with it.
Possibly example could be, going into somewhere or something where she has to rely on someone else to get out (they have to open the door for her, they have to pull her out, etc.) - this would be a very difficult situation for her, because, as is relatively well known, she has trust issues with people, thus having her only exit exist through the actions of another person translates to that same exit/escape route being highly unreliable.
The “figurative” form of this fear can sometimes be seen in how she so far handles personal business.
Estelle does not prefer to be “trapped” by obligations in the business sense and always has a preference to arrangements that appear to have some sort of “open door”. Even in her business with Savage Hammer, part of what had her so attracted to the arrangement (aside from it providing the protection she needed) was its sort of mutualistic nature. She provided services for them, and was basically paid in services in return, with, in reality, relatively little real money moving between the two sides – aside from what she was securing for the team. This, in theory, made it a relatively easy and simple matter for them to go their separate ways should the team fail to meet her needs.