The universe may have an in-built mechanism for causality protection, preventing wormholes specifically from forming time machines.
Wormholes as we understand them don't always form time machines. They can be arranged safely and even into complex chains/networks while remaining stable. The trick is to balance them spatially and temporally.
Let's look at an example where two wormholes form a time machine.
Like everything else, wormholes experience relativistic effects like time dilation. Let's create 2 wormholes, A & B, side by side and synchronized, and send B off to some distant star 100 ly away at near the speed of light, while keeping A here at home. Measuring B's transit with Earth clocks, we'd expect it arrive in 100 yrs. However, according to clocks traveling with B, the trip from Earth to the distant star takes only 1 year. There's nothing strange about that. That's basic time dilation. However, this is a wormhole, and we have the other end point to consider. A and B contain the same set of points in space-time. Looking through A, situated on Earth, we see B arrive in 1 year. After only 1 year of flight, astronauts step through wormhole A and visit the star system 100 ly away.
You might think that's impossible. After all, we can look through a telescope on Earth and still see wormhole B in flight to the star, with 99 ly to go. But that's relativity. If you attempt to catch up to B, you'll never reach it before it reaches its destination. In its frame of reference, the 100 ly distance is length contracted to only 1 ly. In its frame, the star isn't 100 ly away, but only 1 ly, and its frame is just as valid as any other.
Anyway, wormhole B is now 99 years in our past, and any astronaut stepping through arrives 99 years in our past. They will have "time traveled". However, with the current arrangement, there's no way to communicate with Earth before the wormhole's departure. Lightspeed messages through space from the star still take 100 years. The astronauts may be situated -99 years temporally, but they are situated +100 years spatially.
That's the rule. A wormhole's temporal offset must be less than its spatial offset.
If one wormhole endpoint sits 1 second into its counterpart's past/future, then the two cannot come within 1 light-second separation without forming what's known as a closed time-like curve (CTC), which is what time machines are made of.
So, how does the universe exhibit an in-built safeguard?
A CTC is a loop. It is a trajectory through space-time whose endpoint lands on its own start point in both space and time.
Imagine a photon traveling along this path. It travels the entire path and constructively interferes with itself in the past, back at the starting point. The new doubled photon then travels the same path and again constructively interferes with itself in the past, quadrupling, and so on. The photon energy ramps up towards infinity in zero time.
This resonating effect isn't limited to light. According to Matt Visser, "Lorentzian Wormholes: From Einstein to Hawking" (1996), if you look at what happens in semiclassical quantum gravity, you will get a buildup in the amplitude of quantum fluctuations on the cusp of forming the time machine, an effect called "vacuum polarization". These fluctuations are expected to build sufficiently to destroy the time machine, before a CTC can form.
So, when you try to bring two descynchronized wormholes to within their hard spatial-temporal separation limit, this effect destabilizes and destroys the topology, rendering them no longer wormholes. Maybe they dissipate harmlessly as gravitational waves, or maybe they collapse into black holes and dump a few gammas out while they're at it.
Because of strangeness, it's also possible that the wormholes "bounce" off each other, so as to prevent time machine formation.