Tiers always exist, but it's not like someone suddenly decides what decks are there. It's really more of an observation.
Really, Modern only has one Tier 1 deck: Jund. You're basically forced to play Jund or a combo (or combo-ish) deck. Since there are so many "other" decks, it's very hard for them to actually be Tier 1, as they have to share too much of the metagame.
MTGO has a Modern presence, though I believe it's only really popular when it's in season for the GPs and PTs.
If you look up the top decks for recent tournements you can come to your own conclusions on what is top tier. When you look at those one thing is certain; modern is pretty wide open. For example the top ten decks for Grand Prix Lyon were: Jund, UW angels, BUG control, Splinter Twin, 2 Storm decks, Gifts Ungiven, Pod and 2 Robots decks.
You can't actually get the tiers only from decks that finished highly. A deck can be Tier 1, yet have very few recent top 8 finishes. The tiers aren't about the "best" decks, they're about what decks see the most play overall. Usually, these are the same, but if you look at PT RTR, Jund was something like 30% of the field, yet only have a 40% win rate.