Wiring question

Long story short, I installed a votage meter/usb panel in my trailer yesterday. I forgot to disconnect the batttery. Anyway, when I got done with the wiring, I tested it and got nothing. I tested the trailer lights which is what I patched into and some of the lights (LED's) came on and some didn't. But the ones that came on were very low brightness. I couldn't figure it out. If none came on at all, I would have guessed it was a fuse, but since some came on low, I figured it was still getting power. Finally, I checked and sure enough, it was a fuse. After I changed it, all was good.
My question is: Why did some of the LED lights come on partially if the fuse was blown? It was the same with out the fuse at all?