You cannot say what is responsible (if that’s possible at all) but you can say whether climate changes makes events more (or less) probable than they used to be. Such statements are usually based on simulations (ensembles) where the number of occurrences of particular events is counted. An example that‘s easy to understand is probability of precipitation from the weather forecast: if it says that the probability of rain is 72% at a particular place, then it means that rain was observed in 72% of the simulations (which are run with slightly different parameters to account for uncertainty in starting conditions). This is a simplified description but maybe it helps?