It's nearly impossible to answer definitively without looking at your credit report, which understandably you wouldn't post here (please don't!). But a few general comments/suggestions.
First, don't worry about short term changes that are not caused by fraudulent activity, and are not caused by a late payment or other negative activity. If my score dropped by 50 points, I would first make sure the accounts on my credit report were my accounts - that there wasn't a new account that I didn't know about - and I would second make sure there wasn't an account that I was late paying on that I didn't know about. This could include both forgetting to make a CC payment, and also something like a medical bill I didn't realize I owed show up suddenly - that's not uncommon.
Second, particularly when you have a high (800+) score, it's not uncommon for utilization to play a big factor, and utilization can vary from month to month. My utilization is always low, because I have several high balance cards, but it still shifts from anywhere between 0% and 10% depending on when the cards report usage and what I've been buying recently. That causes significant shifts in my credit, largely because it's the only potential negative - I have a reasonably long established history, good history with no dings, etc.
I would see what happens next month, after you pay off the big item you said in comments that you posted recently. If it goes back up - then you know that was the problem. If it doesn't, and the reported balance is lower, then you may want to look at other items (such as Pete noted an unauthorized hard inquiry) with a full credit report pull of all three agencies.
Also, note that you can usually find out what the algorithm considers your 'biggest negatives'; I like to see what those are each month just to make sure they don't change. (For me they're always utilization and length of credit history; neither is meaningfully negative, but they're the most negative. If that changed suddenly, it might be a sign of a problem.)