https://www.ciscopress.com/articles/article.asp?p=3089352&seqNum=6

A late collision is defined as any collision that occurs after the first 512 bits (or 64th byte) of the frame have been transmitted. The usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non- compliant number of hubs in the network, or a bad NIC.
Late collisions should never occur in a properly designed Ethernet network. They usually occur when Ethernet cables are too long or when there are too many repeaters in the network.
Reference: https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html

Lower priority means it is preferred compared to a higher. If there is a tie in priority then the lowest MAC address will determine which bridge becomes the root. Because S2 has the lowest MAC address, S2 becomes the root bridge.