For grammar, I'd really advise anybody to just buy a good book. Although for quick searches the internet is of course a good choice, when it comes to quality, books are still the medium of choice if you ask me.
～つつある also often conveys the notion of gradual change, like in 変わりつつある or なりつつある. Thus it can often be translated as "in the process of ~". With 起こりつつある, there's probably some subject indicating change and it actually becomes something like "... change is in the process of happening", meaning something has begun to change (and you can see/hear/feel/... it) but hasn't reached its final outcome just yet. With a verb like 起こる, this can of course also imply that something has started to keep happening.