Any odd smells? Eggy ones for example? Bubbling substrates tend to be hydrogen sulphide being created by anaerobic bacteria building up in the substrate. Can you get the wood out to have a sniff of it? Give the gravel a bit of a swoosh with a gravel vac and see if anything else bubbles. If that is what it is then it isn't good, I experienced this myself with a manky substrate and it gave the fish all manner of health problems
H2S just means you need to keep the substrate turned over more often, it is created by the activity of anaerobic bacteria. Either stir it up yourself when cleaning or/and get some malaysian trumpet snails.
I think you're probably just seeing the effects of water losing some of its gasses. as you know if you leave a glass of water on the side overnight in the morning it's got bubbles stuck to the glass all the way round. These little bubbles form everywhere but are redissolved and/or escape through the water surface over time so you never normally notice them. In a normal tank that bubble forming under the bogwood would eventually just bob to the surface and be gone. Then a new one would slowly build up. With the closed top design of the edge it just makes the process more obvious.
Keep your eyes open for the problems Cathie and Suey described but I think you'll find that the only fix would be to ensure that the top slopes towards the opening so the bubble can escape when it forms.