Does anyone audit ASICs made by different companies? Such a hardware can contain logical bombs that can harm Bitcoin when particular conditions are met. For example, starting from 250000th block an ASIC can withhold every 2nd nonce that solves a block. This lowers effectiveness of the hardware by 50% while keeps the same hashrate reported. If everyone mines with such ASICs then 51% attack become 25.5% one.
It is not in the vendors' interest to perform a majority attack, but even if they wanted to, there would be better ways to do it anyway.
Firstly, they want happy customers to make more and more sales, they do not want their products to suddenly start working at only half the hash rate, instantly killing their reputation and sales. Do you have any idea how profitable it is to make and sell mining ASICs?
Secondly, if they wanted to perform a majority attack at some point, they would just do it now by keeping some ASICs for themselves.
Thirdly, if someone performed repeated majority attacks on Bitcoin, disproving a fundamental assumption in the system (that no attacker will ever get close to 50%), it would destroy confidence and BTC would lose value, thereby destroying the attacker's own wealth (bitcoins stolen via double spends).
There. That's 3 reasons why a time bomb would be the most stupid thing to do for ASIC vendors.