1. Bamboo is wind pollinated.
2. The flowering (whilst almost simultaneous) actually 'moves' through the population like a wave.
However, what if the plants 'voted' every season by releasing a few molecules of a chemical messenger and once a critical mass of this chemical has been accumulated, those plants then themselves start the same way if they are fit enough to bloom, or send out a different 'nay' chemical messenger that neutralises the 'yea' if they are not fit.
Once enough plants are producing the the 'yea' chemical (and the 'nays' are outweighed) so that a critical mass is achieved, it would trigger the hormone that makes the plant bloom so that the wind pollination has the greatest possible effect. And even if some plants are not ready, the substance when prolific may still have the ability to force flowering, which would explain the uniformity.
In fact, if I was to design such a population and write a simulation for such a model, this is probably the way I'd go about it, because I can think of no better viability test for a wind pollinated species that rarely flowers to ensure optimal success than such a chemical voting system.
Note it doesn't have to be very noticeable, the actual messenger substance may just be a few molecules -- in the same way that human noses can pick out a one molecules of mercaptan in a huge space for example. If we didn't know what smells so bad, it would be difficult to find!