I play a lot at land based casino's and I find the same to be true there - if you cash out it will often trigger something in the machine's RNG and your winning streak will become a losing streak rather quickly. It doesn't always happen like this, sometimes I can cash out 3 or 4 times from the same machine and it always eventually hits another big one. But more often than not, when I cash out, I move on, because I know that it's probably done.
I think the same happens online. Slots will go thru dry spells and runs where you can do nothing wrong. It can change in the blink of an eye and so you need to watchful of the patterns and stop when you're ahead and notice that the wins aren't coming in the same way they were before. I think nearly every gamblers downfall (myself included) is knowing when to stop. We always get that "there's another big one coming" mentality and then when it doesn't come we get angry and look for something else to blame. Random in no way means you're gonna always win, even a little bit, and it also doesn't mean that a lucky streak can't change. In fact, it makes more sense that it will change otherwise to me it wouldn't be random.