Poor man's teardrop I guess?
Reasons might include: increased heat dissipation, slightly increased strength, tapering trace impedance (on this scale, it would only be relevant to >10GHz designs), or uh, not much else I can think of, offhand?
Teardrops are primarily used to connect to thru-holes, where the drill placement tolerance might reduce the pad (annular ring) to zero or negative (cutting entirely through the pad). When this occurs towards the trace, it can weaken or break the connection; the teardrop just adds a little extra meat around the trace to prevent that possibility.
It's not really relevant for SMT pads, but I suppose it would increase strength slightly: tight inside corners are more prone to fatigue and tearing.
Acute angles are also traditionally known to cause acid traps (trapping etchant chemicals, causing eventual corrosion and failure), but this isn't actually a problem on modern processes, AFAIK. Personally, I avoid doing it just because it looks ugly!
Tim