IMHO it began in the early 1990's...With an influx of talent coming over from eastern Europe many N.American trained players found a new challenge in keeping up with their higher levels of skill...Coaches at all levels of hockey caught onto this, and started making changes to the style we see more of today with an emphasis on skating, stick/puck handling, passing, and better defensive awareness.
I think it partially has to do with the change in rules, but i wouldn't put it all on the rule changes. The game is faster than it was in the '80s and '90s, can't clutch and grab like you used to. But i just can't blame the lack of true power forwards on a few rule changes. It's an interesting question.
- T.Low likes this