Is the choice RANDOM...I doubt it...Filipinos are among the most verbally talented people I ever met. How could word affix choice be random?
@BoraMac
Not random, but there IS such thing as a commonly accepted norm. Many times expressions in a language have no "reason" that can be logically deduced for why it is one way and not another...it is that way, simply because that's the way it is. And you have to relax and accept this is the case sometimes. Language has subtlety and intricacies that just cannot be "logicked" about. ; )
For example, in English if you're abruptly quitting cigarettes, you're going "cold turkey." You're not going "cold chicken". ; ) There's no reason why the turkey has the spotlight here, it's just the customary way of saying things.
Going "cold chicken" is unambiguously wrong.
In Japanese you say you "want to go to the bathroom" and in English you say you "need to go to the bathroom." Why the difference? No reason. It's just the norm in their respective languages.
In the same way, sometimes using an -UM- verb when a MAG- verb (or vice-versa) is also required, or wrong.
This is a slightly different example, but in Tagalog "tawagan" is to call someone on the phone, and "tawagin" is to call out to someone. There is no logical way to deduce which is which, why one is an -AN verb and one is an -IN verb. You simply have to memorize. Most verbs are like this in Tagalog. Depending on the context, usually an -UM- verb is require or a MAG- verb is required. Sometimes you can use either, in a limited number of verbs. Sometimes the -UM- version means something different than the MAG- version. Sometimes there are also regional differences.