pump and dump.
which is basically pure manipulation of the market. sometimes it starts with some sort of news which hypes the coin and leads to its pump. for example Stellar was listed on Coinbase (or is going to be listed?) and that hyped it up a lot and lead to a big pump of it.
but sometimes it is just pumpers picking up some random shitcoin and pumping it for short term profit.
And also why some exchanges have higher selling price and others have lower selling price. Can you share your thoughts?
price of altcoins are always the same with a very small difference on different exchanges.
if a difference occurs it will be resolved in a very short time because people either buy/sell that coin on the other exchange and bring the price to the same level as other exchanges or they arbitrage it to come to that level.
if you see a persistent difference then it means there is some problem with that exchange with different price.