Maybe nothing. Perhaps Windows is adding something to the files. A lot can depend on how well you know ABC coding. I use a Linux operating system and had to search around to find ABC programs that will run on my machine. I've got it down to two programs. ABCJ is a free program which only works with a single line of music and Five Line Skink, also free, which can display multi-lined scores. My point is that a file that will run on one of these may not work on the other. I can usually look at the file and see what needs changing. One collection of tunes contains a capital T for trills and one of my two programs fails on these. If I edit out all the3 Ts then the file displays and plays.

Try downloading your files into a text editor, (not a word processor) and see if the program will work on these.

Does abcexplorer highlight errors in the editing window?