Probably in the same way that the price of gold is manipulated through gold futures and ETFs. Basically if TPTB want to suppress Bitcoin's price they would trick the public into buying paper BTC (instead of actual BTC) while they sell the underlying that they have previously hoarded at a lower price.
This could happen if, for example, BTC futures became more accessible than actual BTC to the general investing public. The average InvestorJoe is more comfortable buying a futures-based BTC product through his BAML/Schwab/Fidelity/etc. brokerage account than opening a BTC exchange account at some incompetent startup that is not SSAE16 audited for compliance in protecting customer data. When the current crap exchanges talk about compliance they only think within the scope of AML/KYC compliance, which does not cover the protection the customer's private data at all.