Long story short, they allowed two separate enemies to escape, are extremely low on resources (and have even lost one party member) and are planning on taking a long rest right next to the unsecured teleportation circle leading to the BBEG lair. They have convinced themselves through a circular argument between players that the bad guys will lay in wait for them to come through, ignoring the fact that the second escapee knows they are in their weakest possible state and nothing is stopping a force to come through the portal to attack. One player has been adamant that they should leave the location to rest and go the overland route to the BBEG lair (which is a good idea and would work) but other players have argued him down.
So, teleporting into a circle is a highly dangerous act if the circle is occupied by your enemies.
They could easily have set up a killer ambush, possibly guarded against scrying, hitting whomever shows up. They can even destroy the circle easily during teleportation, splitting your forces if you come in waves.
If the BBEG has any kind of plans that the players are disrupting, then breaking the teleportation link and finishing their plans makes a lot of sense.
An initial scout through the portal is also a good idea. Any scrying on an alert enemy in a plausible ambush spot should be considered suspect; so if they can, they'd scry the location (doesn't even grant a saving throw, but
does open up risk the PCs spot the sensor), confirm a lack of ambush, send through a scouting force (cheap enough to lose, strong/sneaky enough to last a round) via teleportation circle, keep scry up to see if the sensor was faked out, then send a larger follow-up force through next round.
Lacking scry, you do much the same, except you send a scouting force with the ability to know if they are wiped out and/or communicate back (a sending, for example).
Lacking even that ability, BBEG side either (a) set up a killer ambush at their end, (b) send a reconnaissance-in-force through, or (c) break the teleportation circle link.
The BBEG side should assume the other side is clever, even if the PCs are acting dumb.
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Anyhow, as for the result, don't fall into "DM my guy" syndrome.
My guy syndrome is when a player does something that makes the game less fun for everyone, because "their PC would do that". The fact is, their PC's motivations are
within the player's control.
Similarly, as the DM, the BBEG motivations and the like are within your control.
If you want the PCs captured or similar, it isn't "they have no reason to capture the PCs", it is "I cannot think of a reason they would capture the PCs that is consistent with what the PCs know about the BBEG's side and the constraints of my planned plot".
If you want them captured, what I'd do is think back over the last sessions or the PCs backstory or the like, and see if you can invent a connection between something utterly random that happened and a reason to capture the PCs.
Once you have invented that connection, and found something to ground it in, pretend that was the plan all along. Work out a way to highlight that connection -- it could be as simple as one of the BBEG side saying it openly when the PCs are captured -- so the PCs see the reveal.
In the last fight, did they focus-fire PC1 and not attack PC2 that much? Is PC2 some strange race? Turns out that they where under orders to capture PC2, because they need PC2s blood for an experiment. I mean, didn't the players catch that? It was so telegraphed. (Actually, it wasn't true until you needed an excuse to capture the PCs). The capture of the other PCs is collateral damage by some underling who wasn't clear which ones are supposed to be kept alive.
You could even have the BBEG brutally kill one PC (talk to the player first, maybe the one who warned everyone that this was a dumb plan, or someone whose character arc could do with a replacement), and they reroll a new PC that helps them get out of confinement in the BBEG dungeon.
