I saw so many matches when they tied, and result also comes by some very interesting plays, like Super Over, and some of the matches remains tie only. But in First T-20 World cup, when India and Pakistan's scores were level, then both the sides threw some deliveries to the stumps, what kind of rule was that?
Answering your main question:
Why some of matches remains tie and some goes to superover?
The decision of keeping the match tie or go for tie breaker like Super Over depends on a particular series. Mostly, tie breaker is used in the series between more than two countries like any ICC world events, continent tournaments like Asia cup etc.
Regarding your other question (which is already answered by @Sports Fan):
The decision of which type of tie breaker (Bowl out or Super over etc.) should be taken also depends on the series.
Edit: (As suggested by @Sports Fan)
ICC announce such playing conditions before the tournament is started. See this report in thehindu.com.