Luckily there seems to be less attacks in the last half a year.
I think there's been more attacks, but here's a summary of some of them: http://yle.fi/uutiset/3-8517586 (Dec 2015, in finnish)
Look, I'm fully aware you need to panic, and I'm fully aware this need is being satisfied by a bunch of cronies standing to make heaps of money on your panic. Just know your fear is not justified and should not be the reason for aiding the raise of fascism in europe.
What I know is that these grenades came from the Balkans after the civil wars in the former Yugoslavia, now why is it so common for it to happen in Sweden of all countries I honestly don't know.
One theory is that due to a quirk in the way the law is worded, grenades where not considered weapons, but rather a controlled explosive[0]. What this means in practice is that the punishment for smuggling hand grenades into the country is much much lower than the punishment for smuggling in guns. As the law is written, if you are first time offender caught with a hand grenade you're likely to get away with a fine.
[0] https://www.svd.se/fritt-fram-ta-in-handgranater-i-sverige
Semi-related: A significant amount of illegal firearms in Europe (including the firearms used in recent attacks) were sold legally in Slovakia as "deactivated acoustic expansion weapons" and then "reactivated".