Start-ups are afraid of being bought by big companies because they're scared that they might steal or copy their resources (innovations) and then close them off. It would lead the to bankruptcy. Big companies can propose these innovations at a cheaper price.
In fact, if big companies want the start ups' innovations but start ups refuse to sell them,big companies will hire the best engineers of the start ups to compete on the same kind of innovation and make the start up lose their idea. That's why start ups accept the huge amount of money that big companies give them.
All these fears are called "The kill zone".