I think that some people are natural leaders and some are natural followers because personality is likely to determine these characteristics.
I think that the environment could determine people's characteristics and decide whether they should be a leader or a follower.
So I think it is usually naturally decided, but because of the environment, there could be an exception. =  think some people are natural leaders while others are natural followers because their personalities are likely to determine these characteristics. I also think that the environment can influence people's characteristics and determine whether they become leaders or followers. So, I think these roles are usually determined naturally, but there can be exceptions depending on a person's environment. 

CORRECTIONS:

• some are natural followers = others are natural followers
  → The pattern some... while others... sounds more natural.
• personality is likely to determine = their personalities are likely to determine
  → More natural because you're talking about different people's personalities.
• could determine = can influence
  → Influence is more appropriate because the environment affects personality rather than completely determining it.
• decide whether they should be a leader or a follower = determine whether they become leaders or followers
  → More natural and avoids the idea that someone should be a leader.
• naturally decided = determined naturally
  → More natural word order.