What are their motives:
To pump and dump.
Or
Pump and get the price of BTC and investors confidence up. In this instance I would hope the intentions are good!
Not every rise in the price of the bitcoin can be termed as a PUMP in the price and similarly not every decline in the price can be called as a DUMP in the price. It requires a continuous observation of the market to interpret something out of the crypto market. If it is the option II, i.e., Pump and get the price of BTC and investors confidence up, then of course what else is good for the bitcoin community.
No one or nothing is responsible for the price pump of Bitcoin. It's just the nature of it which goes ups and downs. If people buys Bitcoin to a large amount and hold it for long, the price gradually start increasing.
That is pretty much clear to those who have been into this space for quite a long time. It is the market forces of demand and supply that have a direct bearing on the prices of the bitcoin.