There’s a number of reasons that could account for the drop…
People burnt out on t5
People relaxing until sunwell
People relaxing until wotlk
People upset over LFD in Wotlk
People somehow really hyped over dragon retail expac launch and going back to retail
People just leaving dead servers for greener pastures
People rerolling for wotlk chars
People upset over Arena season 3 being super super short
People upset that it looks like S4/Sunwell tier is also going to be short
Or, as a bit of a darker thought, since it was mentioned the drop started before the announcement, how many wow players played from Ukraine before that War began? If the game was at all popular there, you can bet most of them aren’t playing video games right now.
I’d also add there was a new wrath pserver that just opened up as well like a week or so ago… perhaps a bunch of people jumped over to that.