There are two levels we must consider.

First, there are the countries which were directly involved in this "territorial dispute" as of the end of WWI. Naturally, it was in their best interest to keep (in the case of Hungary) or to gain (in the case of Czechoslovakia and Romania) as much territory as possible. There were attempts to settle the issue locally, like the Hungarian military campaign into Czechoslovakia or the Romanian military campaign into Hungary, but eventually the decision was made elsewhere.

 

The second level concerns the victorious great powers, such as France, the United Kingdom and the United States. There were two conflicting directives for the matter.

First, there was the Wilson Doctrine, the idealistic desire to achieve self-determination for each nation. This was partly achieved, in the sense that the numerous unrepresented minorities in the Austro-Hungarian Empire all were either given new states or they were able to join existing states of their respective nationalities. In theory the right for self-determination would have also been provided for Hungarians.

The other directive was largely coming from France, with the heavy "vae victus" (woe to the vanquished) approach, which wanted the defeated countries as weak and as contained as possible. For this purpose, the idea of a cordon sanitaire, a "sanitary cordon" was born (I must note that this term is frequently used in a lot of different contexts, even in this very same era). The idea was to create a France-aligned cordon of new states which would keep the defeated powers in check. This required the new ally states (such as Czechoslovakia and Romania) to be as strong as possible.

Ultimately, the French approach prevailed here, and as a result, the borders were not redrawn among ethnic lines.