So, I ask, what are the trends leading to the brittleness of a metal
Brittle materials absorb very little energy before fracturing. The only trend that can be linked to this is the strength of bonding in the metal (Other factors like crystal structure and purity do not have any trends and so there are a lot of exceptions). This also means that the trend in brittleness is not very regular since it is complex and depends on too many other factors.
why are these trends there? (usually trends can be explained in terms of simpler things like atomic size/sheilding/etc)
Since brittleness cannot be explained by a very simple trend, I will explain what reasons I have found.
The stronger the bonding in the metal, the more difficult it is to change its structure and thus it tends to break instead of deforming. Here's a graph (melting point vs. Group number) that gives a general measure of strength of metallic bonding in all the transition elements of the first three series.
According to the graph, we predict that Tungsten (Period 6 Group VIB) should the most brittle and this is true for impure tungsten. Again, this successfully predicts that Silver, Gold and Copper (Group IB) are least brittle (and so most malleable and ductile). Also Lanthanum (Period 6 Group IIIB) is predicted to be quite malleable and this is also true.
Note that this also explains the high malleability of alkali metals (the graph is not very clear at that point).
There may be much more exceptions to this trend when examined closely so take this only as an approximation.
Crystal structures also play an important role in brittleness but they cannot be explained by a trend.
Purity of a metal tends to increase malleability and decrease brittleness because there are less points at which a fracture can take place.