I can't speak for comic books because I was never really in to collecting them, but for baseball cards-Jay wrote:
The first time was overproduction. Everyone got into collectibles because there were stories about baby boomers finding $100k+ baseball cards and comics in their attics. Comic book stores popped up everywhere. Seeing the interest, card and comic producers flooded the market and crashed all the prices. The 2nd time was just a normal bubble pop. Collectibles are completely dependent on finding the greater fool.
Overproduction had very little to do with the baseball card price crash. The hobby became outdated. Sure there were more card options, but nobody was buying them, let alone collecting them! It was literally because demand dried up. Now baseball card collecting is coming somewhat back in to vogue, but the market is still based on buyers (like Vegas Dave, who is literally setting market prices on some baseball collectables), not supply. Remember when Todd McFarlane paid some obscene amount of money for McGwire and Sosa and Barry Bonds home run balls?
Boomers had a special place in their heart for card collecting because it reminded them of their childhood. Boomers got older and richer and started fetish buying old cards based on nostalgia. That's what the baseball card collector market is. Same as...Transformers, Star Wars, GI Joes, virtually any childhood toy that is now a collectible for Gen X and Millennials.
I mentioned a while ago that I broke a box with a friend. It was the fastest I've ever seen $2000 turn to zero in my life. I have a cool 2009 Bowman Chrome Set Neftali Feliz Refractor card to show for it though...know anyone that wants to buy? To me it's worth $1000, to everyone else it's probably worth around $0.05.