Is it true that when you think of someone all the time it is because they are thinking of you?
-Anonymous
Could be the Holy Spirit places them on your heart to be praying for them.
It is the Holy Spirit that know all things, and He knows all that a person needs in each moment in their life, and He know when they need prayer.
When this happens to me I pray for the person that has been brought to my mind, and when I talk to them later they say that was when they were needing prayer for a certain issue in their life.