Sometimes bubbles just happen.
I get them occasionally. I make sure to shut off ALL equipment when I so a water change, this way, they don't suck in air.
Some systems just get a gulp of air every now and then and it will get stuck in the filter/line and come out at random times. The best thing to do is just watch the system and see if you can spot it. I can usually see whats up after looks at everything "under the hood" for a minute or two.
Algae is a good thing, it's a sign that there is some life and goings on in the tank
Annoying though.