Well those are the main explanations. To add a few details - they are thought to be dry ice geysers.

The dry ice is frozen and transparent or semi transparent, and as it warms up, enough light gets through to warm up the lowest layer of dry ice until it explodes as a geyser.

The same process on Earth creates thin layers of liquid water below ice sheets - it's called the solid state greenhouse effect. It quite probably creates thin layers of trapped water below ice sheets on Mars also - water ice in that case.

But this is dry ice, too cold for the phenomenon to be due to water ice melting, and for that matter, ice gets denser when it melts so wouldn't melt explosively like this, while dry ice gets less dense as it sublimes to the gas phase. So surely the geysers must be created by dry ice.

If it is a geyser effect that is.

Sand geysers could explain mystery spots on Mars

Cosmic Old Faithful: Are There Geysers on Mars? | TIME.com

That doesn't seem too hospitable for life. But the same mechanism - the solid state greenhouse effect (dry ice acting as an insulating and warming layer) - could also perhaps warm it enough to create trapped patches of melted water ice, which life could use.

I think there's also the possibility of life growing directly on the surface, e.g. lichens - maybe the geysers produce nutrients they use to grow on.

I'm quite optimistic that we will find life on Mars.

This is not the top place to look for it - that would be either the warm seasonal flows - or the salt / water ice interface.

But still - I think there is a chance that the dark patches are at least partly created by life processes.

They do look so like life processes! Not a good reason for expecting them to be, I know, but would be cool if they were :).

UPDATE ON THIS ANSWER

I wrote this answer originally some time back - and have found out a lot more since then.

It turns out that though the initial spidery lines are created by CO2 processes, the later stages may well involve liquid water.

These are small features only a few tens of meters in length. And rather than appear explosively in one go, they grow, by a meter up to a few metres per Martian sol through late spring and summer on Mars.

Flow-like features on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence

There are different models for these features in the northern hemisphere (where they happen at much lower temperatures) and in the southern hemisphere.

For the southern features all the models proposed so far involve liquid water at some point. And not just that, one of the most promising models involves liquid fresh water, kept liquid because it is trapped under the ice. Quite thick also for Mars - of order of several cms depth of pure liquid water warmed up by the solid state greenhouse effect of ice above it. The main unknown here is whether or not Mars has clear water ice - the ice that on Earth is blue in colour - on Mars the conditions of formation are so very different, near vacuum and extreme changes of temperature day and night. But if it does have clear ice that the sun can penetrate, then the models show that it should have layers of liquid water below the surface. That then seems a very promising habitat for life on Mars.

I think this idea deserves much more publicity than it has had so far.

Note that this shouldn't be confused with the much more widely publicised RSLs. There are many dark streaks on Mars. Some are formed by avalanches and other wind effects. Some by dry ice blocks flowing down slopes. The RSLs are thought to be formed by some side effect of salty seeps of liquid water flowing down the slope - and they occur only on sun facing slopes when the local temperatures reach around OC  and only on very few such slopes - most sun facing slopes don't have RSLs. So there is something in the local geology that makes them possible.

Anyway nearly all the other habitats proposed there are thought to consist of salty water, probably very salty to be stable for long enough to flow on Mars.

But these spidery dark lines form at much lower temperatures, far too cold for liquid water, and even salty brines. The later flow like features also form at temperatures too low for them to be explained in an ordinary way by water. But the solid state greenhouse effect gives a way for the liquid water to form beneath the ice when surface temperatures are well below zero. And as liquid fresh water. As far as I know this is the only proposed mechanism on Mars to date that could lead to liquid fresh water at above O C on the planet.

For this and many other ideas for possible habitats on Mars, see: Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...