Just like the prices for every other commodity, pricing for housing units (eg rent) is governed by supply and demand.
Between 2016 and 2017, Los Angeles gained ~40,000 people (taking us above 4MM for the first time).
The number of apartments delivered, in the midst of a historic boom in multifamily construction: ~13,000.
Depending on how many people you think fit in an apartment (and my guess is that the median is something like 2), construction is either falling behind population growth by a little or a lot.
Either way, expect more rent growth in the next year or so.