In the pipes analogy, pressure corresponds to potential energy (voltage). Low voltage conditions does not necessarily trigger frequency stability issues.
Grid synchronization is a different issue to the electrical-hydraulic analogy (which, by the way, was the one that Maxwell used and its pretty useful). Grid synchronization comes from the fact that electrical quantities in the network (current, voltage) are alternating periodic. Generators, which are usually mechanical, oscillate and induce such periodic signals. The system must remain synchronized. But this has nothing to do with the network being electrical.
Grid synchronization is a different issue to the electrical-hydraulic analogy (which, by the way, was the one that Maxwell used and its pretty useful). Grid synchronization comes from the fact that electrical quantities in the network (current, voltage) are alternating periodic. Generators, which are usually mechanical, oscillate and induce such periodic signals. The system must remain synchronized. But this has nothing to do with the network being electrical.
[1] https://en.wikipedia.org/wiki/Hydraulic_analogy