My motherinlaw had heart palpitations then a surgery to be monitored and told to stay off her feet then developed a blood clot what could cause this?

Happens with bedrest. Blood clots or deep venous thrombosis can happen at any age but the elderly are at greater risk. I will refer this to a cardiologist.
Stasis. That is a packed question. Assuming you mean the clot, stasis of blood caused by prolonged periods off your feet is a risk factor. Depending on the other issues, bed rest may well have been indicated- it just goes to show that there is no completely benign intervention, even staying off your feet has risks. Clots that are identified promptly can be treated to prevent further harm.