Why do the lug nuts on my truck keep falling off?

I have a ford bronco - lifted with 33" tires. Just noticed a couple months ago that a couple lug nuts were gone - thought it was strange as that has never happened before. I took it down to a local tire center and had the tires rotated and they replaced the missing lug nuts. Well, that was just a couple months ago and I noticed today that I am missing 2 of the 5 lug nuts on one side!! I cant remember if it is the same missing as last time, but what the hell?!?! Did they not get them on tight enough or could something else be the cause? Should I worry about getting 2 lug nuts back on their ASAP or is it ok to wait a couple days? I am a little worried as to why they are coming off...