Quite honestly, it will do you no good to hear other folk's experiences. There are SO many variables that affect the way a dog reacts to a boarding situation.
In your case, you have a pet that you adopted at an adult age. Unless her previous owner dropped her off with a laundry list of documented behaviors and experiences, you really have no clue what her "past life" was like. You've only had her for a few months as well. She may be a well-adjusted dog at home, but we don't know what types (and qualities) of situations you've really exposed her to since adopting her.
Shelters, especially large ones, can be very stressful environments for dogs. There are dogs of so many different dispositions and backgrounds, and it's likely very rarely quiet. Dogs are being walked back and forth past other kennels...the dogs being walked sometimes lunge at other kennels, or dogs within the kennels sometimes lunge at the dogs walking by. This can be a scary situation for an animal that is not used to this type of interaction.
Similar things happen in boarding facilities. Often times, we see dogs react very differently to the boarding environment when compared to their normal, calm demeanor presented at home.
Also, if she's truly a border collie mix, I would not be surprised to witness at least some minor anxiety.
If it's truly concerning to you, then seek out the help of a professional trainer. Perhaps enrolling her in a basic obedience class will help gradually expose her to appropriate, calm animal/people interactions, and the trainer can witness her "aggression"(this word actually has multiple meanings, depending on who you talk to).
