What are the causes of increase in productivity of labour?
In one word: Technology.

It's a bit abstract of course, but it summarises a lot of things very well. It's usually the A parameter in the models we have in growth theory (we augment e.g. Labour by Technology). 
In empirical studies it has been tried to asses productivity by coming up with a measure called TFP (Total Factor Productivity).

A lot of specific things could potentially be subsumed under this broad term. Most importantantly it is the stock of knowledge (e.g. books, blueprints,...) that we improve and build upon. The growth of technology usually depends on the actual stock of knowledge in a certain economy. It can be achieved by Research &amp; Development (in case the economy is on the technology frontier) or simply by technology adoption (e.g. in the case of catch-up economies).

Example: Knowing how to build a tractor, we only need one unit of labour (the guy who drives the tractor) to harvest, say, 100 kilos of grain per day instead of 50 workers that have to cut the same amount by hand. This would be a tremendous increase in productivity because the other 49 people can now do another job in which they potentially add to the economy's output. For building a better tractor (that drives with less fuel, can harvest more, ...) the economy needs the blueprint of the first tractor to build upon, and so on and so forth...