Oh mama, I've been there. My older son, now 4 went through behavior just like this at playgroup for 2 years. Most of the time when he would tackle/bite/hit another child, it would be for no reason and it would come seemingly out of the blue. He wasn't angry. I eventually stopped going to playgroup since he was labled "the bully". It took me a long time to get comfortable being around other children with him, since his behavior was so unsuspecting. This year he has made huge strides, particularly in the last few months. He has not had any incidents. We had to teach him to read other childrens faces for clues that they did not like the behavior he was doing. We had to teach him to tattle when other children hit him, rather than have him hit back. He still loves to wrestle and tackle, but has learned to pick up on cues when they have had enough. It took a long time and there were so many times I felt so sad and lonely. I didn't want my sweet boy with any label attatched to him. Its hard, but we got through it and he is doing awesome.
Be consistent with whatever you do. Teach him how to play gently using dolls and showing him the proper way to behave. Remove him from the situation every time something like that happens.
We use only gentle discipline with our boys and time outs never worked for my older son. So, when this type of behavior happened, we would simply pick up and leave(depending on how bad the behavior was), at the very least, I would remove him from the situation in order to get his attention.