Weird issue with merging videos

[BlueDev]

I got a really weird issue when I was trying to merge two videos. I used the command prompt to do so and that worked out fine. Two videos were merged. However, while the first video had audio, the second one did not! Now if I switched the videos, both videos have sound. In the command prompt, I'd have video1 then video 2 in that order. Video 1 had sound while video 2 did not. I thought why not try switching them? So I put in video 2 first in the command prompt then video 1 in that order and viola! Both videos have sound! I even tried putting only video 2 in the command prompt and it did have sound. I thought that was pretty weird. Why did I have to switch videos in the command prompt to get sound in both videos? I suppose I could do it that way but just wondering why this happened and if someone might be able to shed some light on this?

Thanks.