When a system is rebooted, the sequence is restarted from a negative number again.
This allows recently rebooted systems to be distinguished from systems which have simply looped around their numbering space.
This path can be visualized as a line with a circle at the end; hence a lollipop.
Lollipop sequence numbering was originally believed to resolve the ambiguity problem in cyclic sequence numbering schemes, and was used in OSPF version 1 for this reason.
Later work showed that this was not the case, like in the ARPANET sequence bug, and OSPF version 2 replaced it with a linear numbering space, with special rules for what happens when the sequence numbers reach the end of the numbering space.