my bulgogi fell apart

What cut of meat did you use? And did you cut it, or did the butcher do it?

I recently made Jae Yuk Gui (pork bulkogi) using a pork tenderloin that I had in the freezer. I sliced the meat thin by hand. When I put the meat in the marinade I found it separating, and it was very difficult to lay out for cooking.

Since tenderloin has very little connective tissue, it would probably be better to slice it just a little thicker.

So, I would say the problem was probably your meat.

Sometimes it can happen when you use too much of tenderizing agents(enzymes)

For example,

“Asian Pears” have enzymes that break down proteins,

making it the traditional meat tenderizer,

Pineapples, which are used commonly in Korea nowadays, too have similar enzymes, which are much stronger,

Kiwi (fruit)s have a ridiculously strong protein breaking enzyme

which will literally “melt” any meat into goo.

(So you should never use a lot of kiwis or kiwi juice when your making the marinade

or leave it for a long marinade. )

But, in your case, think the meat itself had some problems,

It might have been some thinly sliced & frozen bulgogi meat,

or the butcher gave you something wrong.

I hope you get a good batch next time.