You cannot do that in a single un-nested for loop.
You can do that with a single un-nested while loop in which you keep a state variable noting which direction you are going in order to figure out what you are trying to do at each iteration.
However, consider
group_start_idx = find(YourTable.tm(1:K-1) > -1, 1, 'last') + 1;
in which case you would process from group_start_idx:K . No loop is required.
If your table is quite big, then it might be worth restricting how far back you go. For example you might have reason to know that there are never more than 1023 negative locations to be grouped together; in that case you could restrict the search
search_from = max(1, K-1023);
group_start_idx = find(YoruTable.tm(search_from:K-1) > -1, 1, 'last') + search_from;
