What kind of reactor media are you using? Do you have a CO2 recirc line?
I'm fairly certain it's a CO2 buildup. The reactor I use right now doesn't have a recirc line and it's prone to this issue as well (I'm tempted to retro one in at some point "when I have some free time" giggle giggle! ... <sigh> but I digress).
Even reactors I've had with a recirc line were eventually prone to this issue. Especially with the older style finer-grained ARM media it would eventually soften enough that the flow through the media would be too resistive and the CO2 would build up. That was always the cue for me to replace my media and it was one of the reasons I actually stopped using a reactor at all for a few years and went to 2-part dosing. The only thing is I don't really like 2 part dosing, the results of coral growth are just better overall with a reactor, at least for me. So here I am back to CO2 and reactors.
But yeah, the unit I have right now always has a huge bubble at the top. I was sort of hoping that coarser media was less prone to this but I bet in enough time it will happen with it as well (especially without a recirc line).