My patreon growth dropped for a bit in june, then it has escalated to a speed i've never seen before since I changed my patreon promo banner (the one I have at the end of episodes).
I think the market will drop and rise naturally during certain times of the year depending on your target audience, but I also believe you can do things to counter that. If you excite people enough and make your tiers affordable (especially in relation to what they get), they are likely to keep pledging unless they hit actual financial problems. You want to give them a deal that makes them feel they would rather spend five bucks on you than on a new tshirt, a box of chocolates or a pizza. Basically make them feel like by pledging to you is a way to pamper themselves.