Massive SERP drop - for what reason?

OK, so a week ago had a 5 page drop on my main keyphrases. No messages on GMT. Normal linkbuilding, a few guest posts some well placed articles nothing huge (all the sites for these were checked via majestic and Ahrefs first and show no issues).
Did acquire an expired domain tha twas a direct competitor (allbeit in the US rather than UK) but had a good trust flow etc with a few good clean backlinks.
So is this just the dance happening? Haven't had a dance that aggressive in a few years. Or is somethign else going on.
Anything else you need let me know - input would be really useful.

what no-one? I know you are all infinitely more experienced than I, but a little advice may help me lear na little more from you

This happened to me as well. I waited for a week or two to see if it would fix it self, but it didn't. However, after a month passed, my rankings returned back to normal, so I recommend just waiting a bit longer, as you shouldn't have any problems with your rankings suddenly without any link building.

thanks for your help, its greatly appreciated