Dota 2

It's what is referred to as "the meta." As heroes get buffed, nerfed, changed, etc. the meta evolves. If Hero X and Hero Y are really good at doing something and Y gets a buff, maybe Y starts to get preferred. Now you know what was really bad vs X? Hero Z. Now hero Z's weaknesses aren't so obvious because Hero X just isn't strong enough to stand against all these heroes that are now being picked.

Eventually a couple months goes by and players have figured out the end of that X>Y>Z chain and a meta is born.

Now usually there are a few large tournaments each patch. During these tournaments we already know the strong heroes, but what about hero/team combinations? What about some cheese/pocket strat that a captain has cooked up? Sometimes certain players or teams just elevate a hero to broken status. It just works better than anywhere else. Well now the opponents have to decide "do i ban Y and Z and leave A in? Other team makes A just way too good, so ill leave Y and Z in play. Oh that worked really well, now how do I counter what else is being picked.

Eventually a new patch (more buffs, nerfs, changes) is released and everyone starts from 0 again. Sometimes (rarely) heroes get so overhauled they remove them from competitive play in order to get them somewhat balanced.

It's kind of late so I didn't do a great job explaining