All the other answers that "no, there is no triggering event, it just happens, quantum mechanics is like that" are perfectly right.
But let's look at the experimental evidence for these answers. Yes indeed, there is considerable experimental evidence that heavily falsifies the idea that there is a triggering event.
This evidence is the statistical probability density for the decay lifetime. It is found in countless experiments that the decay times are exponentially distributed. The exponential distribution is the unique continuous pdf with the following property: if we take a particle which we know has not decayed after some time, any positive time (even if it is a million times longer than the mean decay lifetime), then the probability distribution, conditioned on this knowledge, of the particle's lifetime after that point is exactly the same as the unconditional distribution for the particle's lifetime. The discrete analogue of this dense statement is the geometric distribution and the discrete version of the idea can be summed up as "a coin has no memory". That is, if you toss a coin known to be fair and get a long run of heads, the probability that the next throw will give a tail (or head) is still $1/2$.
If there were internal "clockwork" to the particle that meant there were several stages, separated by "trigger events", to the decay, then we would not see the exponential distribution. Suppose, like a fluorophore, that the particle rises into a radiative state, decays to a metastable state and then fluoresces. If the radiative state's lifetime is significant compared with the metastable state's, then the distribution of fluorescence lifetimes would be the convolution of two exponential distributions, not the exponential distribution. This is indeed what we do see if we examine fluorescence lifetimes carefully. The fluorophore has a "memory" and is like a three-state finite state machine.
I explain these ideas more in my answer to the Physics SE question, "Are There Old Aged Particles".