Yes the cards do play themselves, but there's betting patterns and tricks they use to put their opponents at an unfair disadvantage.
I'll give an example of what happened to me one time.
I was playing an SNG (don't remember the buy-in amount or site I was playing at- happened a long time ago), and got off to a great start. This SNG paid top 3, with 9 entries. My goal was to coast my way into a top 3 finish, and I was well on my way.
Eventually, there were just 4 of us remaining. I believe I was 3rd overall in chips, and had quite a chip advantage over the 4th place opponent. I'd noticed throughout the match that the three of them were very methodical in their play patterns. I noticed many times where the lowest stacked player would raise before the flop (he was to my right), I would fold, and both the other two players would call. That player would again bet on the flop, and be called by both players. Then on the turn, another round of betting would occur with both of the others calling. Then EVERY TIME at the river, this opponent would go all-in, and both of the other players would fold. It didn't matter how many chips the low-stacked opponent had. He or she may have had 60 left, and the pot may have had over 2000 chips, yet both opponents wouldn't call 60 more!
This occurred frequently with just us 4 remaining, and my lead over the low-stacked opponent began to whittle away as a result. I knew what was going on. They were working together to eliminate me 4th, and they would then all finish in the money. I knew if I continued to let this pattern play out, that I would be the low-stack, and they could all work together to blind me down, and ultimately knock me out.
Because of this, I knew I had to do whatever it took to eliminate this low-stacked opponent before they all three could eliminate me. I knew I had to make a stand, and soon. Then the opportunity to do so presented itself. This low-stacked opponent was the big blind (I'm not sure what the blinds were at this point), and I was UTG. I had a pair of 9's if I remember right, and raised 3x the big blind. I was hoping to go heads-up with this opponent (I had him slightly covered by a few hundred chips), and figured I'd take my chances to eliminate him or her now with 9's (if they re-raised me) versus letting my fate be determined by their collusion.
The opponent to my left called my raise, and the small blind re-raised me the minimum. I figured the big blind would fold the hand in hopes I'd be eliminated, but INSTEAD re-raised! While I wasn't thrilled about being re-raised twice, I knew it was time to force this low-stacked opponent all-in, and either take down a top 3 finish, or lose trying! I raised enough to put this opponent all-in. The opponent to my left went all-in (they had me covered)! Then the small blind folded. The big blind only had 500 or so chips remaining. It's a pretty safe assumption to believe that this opponent would go all-in due to the amount of chips in the pot, and how much more it would cost them to play. Also the fact that they re-raised two raisers (myself and the small blind) made that the obvious decision.
So what did this opponent DO INSTEAD? THEY FOLDED! It then occurred to me what had happened! It was a strategic ploy to get me committed to the hand (pot odds). Even if I chose to fold the hand, I only had 200 or so chips remaining, and would be forced all-in the very next hand as the big blind (the low-stacked opponent would be the small blind). Because of this, I had no choice but to take my chances with 9's. I called my remaining chips, and was up against A's! The Aces held, and I was eliminated 4th!
This opponent had disguised his hand by just calling, and the other two players knew that my objective was to eliminate the low-stacked opponent so that I could finish top 3! They bet just enough to force me to make my play, and in doing so I forced myself to play the hand. Who knows exactly what hands the low-stacked opponent (big blind) and the other opponent (small blind) had. Then again, it doesn't really matter. They colluded with each other to get all my chips in the pot, and they knew unless I had A's, that I would be a huge underdog to win the hand.
After this happened, I stopped playing SNG's for a long time. Just recently have I considered playing them again (a MTT SNG at AP looks appealing), but still haven't due to this situation I've just shared.