Cryptos can definitely be manipulated, in a ton of ways. One of the most obvious ways pointed out is pre-mining. The developers pre-mine the currency, giving themselves some large amount of tokens at the genesis block. What this does it allows them to hold for a while, and let the market cap rise. Since a lot of speculators make decisions based on market caps, the pre-mined coins which are temporarily off the market, but included in the market cap, make a huge difference. Then, they can slowly sell off their holdings, and make their money.

Another big way is the "Pump-and-Dump" strategy. Find a small cypto, with a rather small trade volume, and quickly buy a lot of it. The small trade volume means that this unexpected purchase will increase the price a lot. This, in turn, will increase the market cap, and also show a rising price chart. Other speculators will latch onto this as a sign that the crypto went "viral" or whatever, and buy into it also. Then, the original pumper sells his holding much slower, and the higher volume from the new speculators masks this, and he makes out with a bunch of money.

Pretty much anything that affects market caps, quickly changes the price, or otherwise advertises the crypto can be seen, in part, as manipulation. This includes hard-forks, which can be a powerful way to manipulate a crypto if the miners aren't sufficiently decentralized. Sometimes, even if they are decentralized, with every miner game-theoretically teaming up to pump and dump together.

That said, manipulation isn't the end all. A crypto can still be a good speculation even with manipulation. Any person holding the crypto can "latch on" to the manipulation and ride the waves along with the manipulator too. And probably every crypto has experienced manipulation, to some extent, so good luck avoiding manipulation itself.

At the end of the day, manipulation isn't so much a bad thing, just something to recognize. Before buying into something, look at it for the signs. Ask why the market cap is what it is. Why did the price increase when it did? How long has it been since it last rallied? Is it likely someone will want to manipulate it in the future? These questions, and more, are wha you should be asking yourself before coming to any decisions.