The shown code creates the file "1.jpg", "2.jpg" and so on. After getting the file names by dir they are ordered alphabetically, which is not the original order. Solve this by:
Folder = 'I:\testing\abrupt\';
for iFrame = 1:n
frames = read(vid, iFrame);
imwrite(frames, fullfile(Folder, sprintf('%06d.jpg', iFrame)));
FileList = dir(fullfile(Folder, '*.jpg'));
for iFile = 1:length(FileList)
aFile = fullfile(Folder, FileList(iFile).name);
img = imread(aFile);
fullfile cares about file separators. Now changing the work folder requires a single change in the code only.
"iFile" and "iFrame" is more descritpive than "i", which might shadow the imaginary unit also.