What could cause my Macbook Pro to turn off after just starting up?

So, I recently got El Capitan to install on my 2009 Macbook Pro. Everything worked fine. I used it to watch youtube, surf the web for a bit, and I messed around with the settings to my liking. Well, I turned it off to run up to the store. I came back and had turned it on. After having turned on, it booted up like normal but turned immediately off. I did it again and the light came on, then turned off. I did it again and it booted up to a white screen with the Apple logo and a status bar but then turned off. I did it once more and it booted up as normal and am now using it with no issues. That's how I'm writing this, on the Macbook. I'm just curious as to what the issue could be. Perhaps the age? I just replaced the logic board and ram, along with the hard drive cable. As long as it turns on and works eventually, I'm not bothered by it but don't want to ruin it by it constantly crashing essentially. Any thoughts? Thanks, much appreciated!