I sometimes experience muscle twitching, especially on my non-dominant (left) side. I usually use dumbbells or cable machines when I lift and I perform one-sided movements: weak side first, followed by my stronger side. I occassionally do more reps with my left side in order to make it as strong as my right. My theory is that sometimes I overwork that side and that it remains hyperexcitable for a while. The twitching usually happens in either my bicep or sometimes my tricep.
It’s kinda weird, watching the muscle move without my brain’s permission.:)