All of the above. Most likely are a tight link on the new chain where you joined the chain together, less likely if you used a master link. But RG has it right: suspend
the bike and slowly turn the crank while watching the chain go through the RD. OTOH if the problem happens while riding, ie chain under some torque then new chain/
worn cassette is the likely culprit. Generally cassettes last two chains. Worn idler pulleys possible but it generally takes a lot of riding to do that, depends on how
they look, should have relatively square teeth. Also top pulley should 'float' sideways on its bearing, but the bottom pulley does not, if inadvertently reversed
somehow it might cause a problem.