I actually work in a lab and take blood from patients. I don't do the testing, but I am familiar with some of the reasons why patient's liver enzymes can be high. Many times it's the medications they are taking, such as for for cholesterol, or even acne medications, but there are many others as well. If you are only taking Zoloft or any anti-anxiety/depression medication, as far as I know, it shouldn't affect your liver enzymes. Alcohol and diabetes can also elevate your liver enzymes among other ailments.
I'm sure you've searched the net for answers, since your doctor doesn't seem to know why either, but a good site I found is www.mayoclinic.com/health/elevated-liver-e
and I'm sure there are many, many others out there as well. You've probably already come across that site since it's a popular one.
Regarding the biopsy, I guess it's to rule out anything serious. At least you'll be sedated and it should be a simple procedure but that doesn't mean you won't get anxious about it. I know I would. I guess that's where CBT comes in, think positively, breathe, try to take care of yourself and treat yourself a bit (not with food necessarily!). I've recently uploaded (we can still do that legally in Canada) some sounds of nature or other meditation tracks. I find the sounds of rain and thunderstorms very relaxing. Maybe you can listen to some favorite music or nature sounds for example when you feel really stressed and anxious. It really does seem to help, at least for a while but that's better than nothing!
I wish you luck and I hope it's nothing serious. Feel free to keep asking for advice or even venting about your anxieties here. I sure will and do.
Hope that helps and feel free to e-mail me if you have any personal questions you do don't want to post here. I can't promise that I'll be able to answer them, but I'll do my best.