They do that sometimes.
In all probability, two calves have slipped and the cows have returned to heat.
But there's enough chance of them doing this while pregnant that I would first look for other signs of pregnancy, then get them preg-checked if still uncertain, before doing anything drastic like culling them.
Cows on the point of calving being ridden is fairly common. Occasionally cows have apparent standing heats while pregnant, either every three weeks or just a one-off to perplex their owner.
Also possible that the cows were not cycling and their hormones have finally sorted themselves out - got one in my herd obviously not going to calve, her last cycle would be about ten months ago.