Their languages are from the same family. In other words, it's easy for Romanians to learn Spanish. Also, it's an easy access country for them in comparison to the options of similar or better quality.
Similar cultures, languages, climates. After Communism ended in Romania and Romanians were looking for a place to go with a better economy not too different or far away, Spain was an attractive option. By 2007 when Romania joined the EU, making emigration much simpler, there was already a large Romanian community in Spain, and this attracted more immigrants even as the Spanish economy started to worsen significantly after 2008. Once a large expat community is established somewhere, it tends to grow as people from the original country come to join friends or family members who previously emigrated.