A bit of both actually but the main 'cause' is equipment, no doubt about it. Bigger and better drivers have allowed golfers to develop a technique that is far more powerful and accuracy of impact is not as important as it was say 15 years ago when drivers where 260ccm - they are almost double that now.
The golf balls also go further ...
The golf courses are in better shape and modern US courses are designed with big hitting in mind, creating more opportunity to rip the driver.
But - I know that this is going to cause a storm - I don't many of the modern day players (the last 5 years) could drive it nearly as far with a first first Bertha driver and an 1990's
Titleist 90 Wound. In fact, I'd put money on that someone like Bubba or JB couldn't even get a centre hit with thier current swings.