Thanks to the privacy offered by Monero it is hard to know the purpose of most transactions.
It is likely that none of the above suggestions are currently the largest driver in transaction volume.
Based on trading volumes and the number of Monero accepting merchants today, a large percentage of transactions are likely related to trading. This can include:
- Sending Monero to exchanges
- Sending Monero from exchanges to web wallets, CLI wallets and paper wallets
Trading volume are often the highest around the times of major releases (LMDB release) and major events (ETH/ETH drama when immutability was often discussed).
In addition to major releases, I would expect transaction volume to show big spikes when major privacy focused merchants accept Monero. If many Bitcoin users became aware of xmr.to as a private way to pay Bitcoin merchants anonymously I would expect an immediate spike in Monero transaction volume.
If more trading moves to P2P exchanges such as Bitsquare where XMR is traded on chain (instead of off chain at centralized exchanges) transaction volume should increase substantially.