Should I apologize to my coauthors for being slow to finish a paper draft?
Delays on this sort of thing are extremely common.  So, unless there is some specific reason that this work is time sensitive, it's unlikely that anybody is going to be surprised or upset by this: don't worry too much about it.  Certainly, don't beat yourself up about it, especially when you had legitimate reasons for the delay.

That said, a brief apology is a good idea.  All you need is something like:


  Sorry for the delay with this; the last few months have been really hectic.


Don't make too much of it; just acknowledge the delay and move on.