why are volcanic erruptions always accompanied by heavy rain ?

Answer 1

If that is the case, it is due to the dumping of large amounts of particulates (dust and ash) into the atmosphere, which then can act as nuclei for the enhanced formation of clouds and rain.

In addition there might be some cooling locally by the ash and dust cloud itself, which could in turn, cause air temperature to drop below the local dew point and enhance the possibility of rainfall.

Finally, as it turns out, most large, active volcanoes are in geographic areas that already receive high rainfall: they are mainly along mid ocean ridges and along continental margins. There are many exceptions, of course.

Answer 2

There is no known correlation between volcanic eruptions and heavy rains. None.

Maybe you refer to volcanic mudslides known as lahars that often occur during eruptions. If the volcano has a considerable amount of ice on its peak, when it erupts the ice melts and mixes with ash and debris and triggers a mudslide that moves at high speeds down the slopes of the peak leveling anything in its path.

Notable cases of eruptions with lahars are Mt. St. helens, Nevado del Ruiz and more importantly Pinatubo in 1991.

Answer 3

They aren't. Volcanic eruptions just produce large clouds of sulphur and ash - along with light/porious rocks like Scoria and Pumice.