What you might have is your water supplier adding more disinfectants (chlorine, chloramine) than normal. They will do this for a variety of reasons, as their main concern is supplying a safe product for human consumption. Unfortunately this isn't good for fish, especially if you are unaware of them doing this.
The product you're using to treat the water is a good one, converting the ammonia that comes from splitting chloramine into its components to ammonium, but perhaps your bio filtration is taking a bit of time to convert this from nitrite to nitrate, thus the spike.
The biggest problem is often getting the correct information from your water supplier as to when they plan on doing this, if they do plan it. The best thing you can do s learn about your water supply, and what they may do in certain instances, this is often weather or demand related. Once you figure this out you can adjust the amount of dechlorinator accordingly. I'll usually double dose my tanks, though at other times a regular dose is often more than enough.
