Why am I getting beer volcanos?

Hi- so I have been back to home brewing for a little more than a year- and lately I have been experiencing 'beer volcanoes'- for my dark beers I dare not turn my back on them when I open them because for too many of them, the moment I do, they staring erupting.

It is not happening with my light ales or lagers- just with my big Belgians or big darks.  For my priming sugar I use the same amount each time for my 5 gallon batch- the premeasured corn sugar sold by my brew supply store.

Could it be that I am not letting my big beers ferment out completely enough so that there are still fermentables when I add my sugar for bottling?

Any suggestions on how to handle the volcanoes I already have- they are amusing at first but I would prefer to drink that beer....

When I've had gushers, it was always a result of an infection. Take one of the bottles of dark stuff, hold it to a bright light, and look for a ring at the fill level. 

If it is consistently happening with the higher gravity beers, then you are most likely not allowing it to finish fermenting.  Do you measure FG on your batches versus target? 

If they are stalling out early, you should also look at your yeast pitching rate  If you are not building starters for gravities over 1.060, you may be under pitching and stressing the yeast out too much, causing them to finish before all the fermentable sugars have been consumed. 

If you look back and see that it is happening to lighter beers and lower gravity brews occasionally as well then you may have a systemic infection which is causing the gushers as Maine suggested.

Greetings - the suggestions from Oginme and Maine are both good.  My only recommendation would be to avoid using any pre measured priming sugar.  My experience has been that each beer style and type will require a different quantity of priming sugar.  There is no "one size fits all" approach.  I would recommend getting yourself a good quality scale and measuring your priming sugar yourself.  You can get a very good precision scale from Amazon for under $30.  it will also come in handy if you need to weigh your hops.

Regarding the volcanos, my guess is the brew was not fermented all the way or the priming sugar was not evenly distributed.  If you introduced your priming sugar into your bottling bucket and stirred it in thoroughly, it should be distributed evenly.