I think it's just that you ran out of memory while activating the plugin.
On shared hosting accounts, they restrict your memory usage.
I didn't think that affected me as much as it does until I was running WordPress locally one day and although my memory was set just fine, I kept trying to activate certain plugins and they crashed because of being out of memory. It was shocking. Then I started to realize that I was being heavily throttled whenever I was doing a function in WordPress that took too much memory.
Like you, it would usually go through, but I would be waiting and just see it happened when I refreshed the page. For what it's worth, if the plugin shows activated, you should be fine.
Now I am selling WordPress friendly hosting, making lemonade out of lemons :)