The most likely explanation for the drop is simply that the novelty has worn off. In its first two days, Threads received a huge amount of press attention and was inundated with people checking it out. Not everyone decided to make it a daily habit, as you would probably expect — and it likely felt especially alien to Instagram users who had never tried Twitter, but were tempted to log in with their existing account.
https://uk.sports.yahoo.com/news/threads-usage-dipping-already-why-114421177.html